summary and translation

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Summary and translationROC van Amsterdam Gooi en Vechtstreek


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Inleiding

Binnen de ICT is Engels de meest gebruikte taal. Vandaar dat het noodzakelijk is dat je begrijpt wat je leest in het Engels.

Het verschil tussen een vertaling en een opsomming is dat een vertaling veel uitgebreider is.

Opdrachtdoel

Voor het project summary and translation is het de bedoeling dat je deel 1 vertaalt naar hetNederlands. In Deel 2 maak je een opsomming (summary) in het Engels. In Deel 3 maak jeeen opsomming in het Nederlands.

Product

1. Plan van aanpak aan de hand van de 7 wijze Ws. (fase I)

2. Deel 1 is de vertaling van DNS in je eigen woorden. (fase II)

3. Deel 2 is de opsomming (summary) in het Engels van IPConfig. (fase III)

4. Deel 3 is de samenvatting in het Nederlands over DHCP technologie. (fase IV)

Rapportage

Laat fase I t/m IV hier onder aftekenen door je projectbegeleider

Fase I

Fase II

Fase III

Fase IV


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Opdrachtbeschrijving

DEEL 1How Domain Name Servers Workby Marshall Brain If you spend any time on the Internet sending e-mail or browsing the Web, then you use domainname servers without even realizing it. Domain name servers, or DNS, are an incredibly importantbut completely hidden part of the Internet , and they are fascinating. The DNS system forms one of the largest and most active distributed databases on the planet. Without DNS, the Internet wouldshut down very quickly.

When you use the Web or send an e-mail message, you use a domain name to do it. For example,the URL "http://www.howstuffworks.com" contains the domain name howstuffworks.com . So doesthe e-mail address "[email protected]."

Human-readable names like "howstuffworks.com" are easy for people to remember, but they don't

do machines any good. All of the machines use names called IP addresses to refer to one another.For example, the machine that humans refer to as "www.howstuffworks.com" has the IP address70.42.251.42 . Every time you use a domain name, you use the Internet's domain name servers (DNS)to translate the human-readable domain name into the machine-readable IP address. During a day of browsing and e-mailing, you might access the domain name servers hundreds of times!

In this article, we'll take a look at the DNS system so you can understand how it works and appreciateits amazing capabilities.

DNS Servers and IP AddressesDomain name servers translate domain names to IP addresses. That sounds like a simple task, and itwould be -- except for five things:

There are billions of IP addresses currently in use, and most machines have a human-readable name as well.There are many billions of DNS requests made every day. A single person can easily make ahundred or more DNS requests a day, and there are hundreds of millions of people andmachines using the Internet daily.Domain names and IP addresses change daily.New domain names get created daily.Millions of people do the work to change and add domain names and IP addresses every day.

The DNS system is a database , and no other database on the planet gets this many requests. No

other database on the planet has millions of people changing it every day, either. That is what makesthe DNS system so unique.

IP Addresses To keep all of the machines on the Internet straight, each machine is assigned a unique addresscalled an IP address . IP stands for Internet protocol , and these addresses are 32-bit numbers normally expressed as four "octets" in a "dotted decimal number." A typical IP address looks like this:

70.42.251.42The four numbers in an IP address are called octets because they can have values between 0 and 255(28 possibilities per octet).

Every machine on the Internet has its own IP address. A server has a static IP address that does notchange very often. A home machine that is dialing up through a modem often has an IP address thatis assigned by the ISP when you dial in. That IP address is unique for your session and may be
http://www.howstuffworks.com/about-author.htm#brainhttp://www.howstuffworks.com/about-author.htm#brainhttp://www.howstuffworks.com/about-author.htm#brainhttp://www.howstuffworks.com/email.htmhttp://www.howstuffworks.com/email.htmhttp://www.howstuffworks.com/email.htmhttp://computer.howstuffworks.com/internet-infrastructure.htmhttp://computer.howstuffworks.com/internet-infrastructure.htmhttp://computer.howstuffworks.com/internet-infrastructure.htmhttp://computer.howstuffworks.com/bytes.htmhttp://computer.howstuffworks.com/bytes.htmhttp://computer.howstuffworks.com/bytes.htmhttp://computer.howstuffworks.com/web-server.htmhttp://computer.howstuffworks.com/web-server.htmhttp://computer.howstuffworks.com/web-server.htmhttp://computer.howstuffworks.com/modem.htmhttp://computer.howstuffworks.com/modem.htmhttp://computer.howstuffworks.com/modem.htmhttp://computer.howstuffworks.com/internet-infrastructure1.htmhttp://computer.howstuffworks.com/internet-infrastructure1.htmhttp://computer.howstuffworks.com/internet-infrastructure1.htmhttp://computer.howstuffworks.com/internet-infrastructure1.htmhttp://computer.howstuffworks.com/modem.htmhttp://computer.howstuffworks.com/web-server.htmhttp://computer.howstuffworks.com/bytes.htmhttp://computer.howstuffworks.com/internet-infrastructure.htmhttp://www.howstuffworks.com/email.htmhttp://www.howstuffworks.com/about-author.htm#brain


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different the next time you dial in. In this way, an ISP only needs one IP address for each modem itsupports, rather than for every customer.

If you are working on a Windows machine, you can view your current IP address with the commandWINIPCFG.EXE(IPCONFIG.EXEfor Windows 2000/XP). On a UNIX machine, type nslookup along witha machine name (such as "nslookup www.howstuffworks.com") to display the IP address of the

machine (use the command hostname to learn the name of your machine).For more information on IP addresses, see IANA.

As far as the Internet's machines are concerned, an IP address is all that you need to talk to a server.For example, you can type in your browser the URL http://70.42.251.42 and you will arrive at themachine that contains the Web server for HowStuffWorks. Domain names are strictly a humanconvenience.

Domain NamesIf we had to remember the IP addresses of all of the Web sites we visit every day, we would all gonuts. Human beings just are not that good at remembering strings of numbers. We are good atremembering words, however, and that is where domain names come in. You probably havehundreds of domain names stored in your head. For example:

www.howstuffworks.com - a typical namewww.yahoo.com - the world's best-known namewww.mit.edu - a popular EDU nameencarta.msn.com - a Web server that does not start with wwwwww.bbc.co.uk - a name using four parts rather than threeftp.microsoft.com - an FTP server rather than a Web server

The COM, EDU and UK portions of these domain names are called the top-level domain or first-leveldomain . There are several hundred top-level domain names, including COM, EDU, GOV, MIL, NET,ORG and INT, as well as unique two-letter combinations for every country .

Within every top-level domain there is a huge list of second-level domains . For example, in the COMfirst-level domain, you've got:

howstuffworks , yahoo, msn, microsoft, plus millions of others...Every name in the COM top-level domain must be unique , but there can be duplication acrossdomains. For example, howstuffworks.com and howstuffworks.org are completely differentmachines.

In the case of bbc.co.uk, it is a third-level domain. Up to 127 levels are possible, although more thanfour is rare.

The left-most word, such as www or encarta , is the host name . It specifies the name of a specificmachine (with a specific IP address) in a domain. A given domain can potentially contain millions of host names as long as they are all unique within that domain.

Because all of the names in a given domain need to be unique, there has to be a single entity thatcontrols the list and makes sure no duplicates arise. For example, the COM domain cannot containany duplicate names, and a company called Network Solutions is in charge of maintaining this list.When you register a domain name, it goes through one of several dozen registrars who work withNetwork Solutions to add names to the list. Network Solutions, in turn, keeps a central databaseknown as the whois database that contains information about the owner and name servers for eachdomain. If you go to the whois form , you can find information about any domain currently inexistence.

While it is important to have a central authority keeping track of the database of names in the COM(and other) top-level domain, you would not want to centralize the database of all of the information

in the COM domain. For example, Microsoft has hundreds of thousands of IP addresses and hostnames. Microsoft wants to maintain its own domain name server for the microsoft.com domain.
http://computer.howstuffworks.com/framed.htm?parent=dns.htm&url=http://www.iana.orghttp://computer.howstuffworks.com/framed.htm?parent=dns.htm&url=http://www.iana.orghttp://computer.howstuffworks.com/framed.htm?parent=dns.htm&url=http://www.iana.orghttp://computer.howstuffworks.com/'framed.htm?parent=dns.htm&url=http://webopedia.internet.com/TERM/F/FTP.html%27http://computer.howstuffworks.com/'framed.htm?parent=dns.htm&url=http://webopedia.internet.com/TERM/F/FTP.html%27http://computer.howstuffworks.com/'framed.htm?parent=dns.htm&url=http://webopedia.internet.com/TERM/F/FTP.html%27http://computer.howstuffworks.com/'framed.htm?parent=dns.htm&url=http://www.bitmedia.com/cc/url1.htm%27http://computer.howstuffworks.com/'framed.htm?parent=dns.htm&url=http://www.bitmedia.com/cc/url1.htm%27http://computer.howstuffworks.com/'framed.htm?parent=dns.htm&url=http://www.bitmedia.com/cc/url1.htm%27http://computer.howstuffworks.com/'framed.htm?parent=dns.htm&url=http://www.networksolutions.com/en_US/index.jhtml%3F_requestid=196029%27http://computer.howstuffworks.com/'framed.htm?parent=dns.htm&url=http://www.networksolutions.com/en_US/index.jhtml%3F_requestid=196029%27http://computer.howstuffworks.com/'framed.htm?parent=dns.htm&url=http://www.networksolutions.com/en_US/index.jhtml%3F_requestid=196029%27http://computer.howstuffworks.com/'framed.htm?parent=dns.htm&url=http://www.internic.net/regist.html%27http://computer.howstuffworks.com/'framed.htm?parent=dns.htm&url=http://www.internic.net/regist.html%27http://computer.howstuffworks.com/'framed.htm?parent=dns.htm&url=http://www.internic.net/regist.html%27http://computer.howstuffworks.com/'framed.htm?parent=dns.htm&url=http://www.networksolutions.com/cgi-bin/whois/whois%27http://computer.howstuffworks.com/'framed.htm?parent=dns.htm&url=http://www.networksolutions.com/cgi-bin/whois/whois%27http://computer.howstuffworks.com/'framed.htm?parent=dns.htm&url=http://www.networksolutions.com/cgi-bin/whois/whois%27http://computer.howstuffworks.com/'framed.htm?parent=dns.htm&url=http://www.networksolutions.com/cgi-bin/whois/whois%27http://computer.howstuffworks.com/'framed.htm?parent=dns.htm&url=http://www.networksolutions.com/cgi-bin/whois/whois%27http://computer.howstuffworks.com/'framed.htm?parent=dns.htm&url=http://www.networksolutions.com/cgi-bin/whois/whois%27http://computer.howstuffworks.com/'framed.htm?parent=dns.htm&url=http://www.networksolutions.com/cgi-bin/whois/whois%27http://computer.howstuffworks.com/'framed.htm?parent=dns.htm&url=http://www.networksolutions.com/cgi-bin/whois/whois%27http://computer.howstuffworks.com/'framed.htm?parent=dns.htm&url=http://www.internic.net/regist.html%27http://computer.howstuffworks.com/'framed.htm?parent=dns.htm&url=http://www.networksolutions.com/en_US/index.jhtml%3F_requestid=196029%27http://computer.howstuffworks.com/'framed.htm?parent=dns.htm&url=http://www.bitmedia.com/cc/url1.htm%27http://computer.howstuffworks.com/'framed.htm?parent=dns.htm&url=http://webopedia.internet.com/TERM/F/FTP.html%27http://computer.howstuffworks.com/framed.htm?parent=dns.htm&url=http://www.iana.org


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Similarly, Great Britain probably wants to administrate the uk top-level domain, and Australiaprobably wants to administrate the au domain, and so on. For this reason, the DNS system is adistributed database . Microsoft is completely responsible for dealing with the name server formicrosoft.com -- it maintains the machines that implement its part of the DNS system, and Microsoftcan change the database for its domain whenever it wants to because it owns its domain name

servers.Every domain has a domain name server somewhere that handles its requests, and there is a personmaintaining the records in that DNS. This is one of the most amazing parts of the DNS system -- it iscompletely distributed throughout the world on millions of machines administered by millions of people, yet it behaves like a single, integrated database!

The Distributed SystemName servers do two things all day long:

They accept requests from programs to convert domain names into IP addresses.They accept requests from other name servers to convert domain names into IP addresses.

When a request comes in, the name server can do one of four things with it:

It can answer the request with an IP address because it already knows the IP address for thedomain.

It can contact another name server and try to find the IP address for the name requested. Itmay have to do this multiple times.

It can say, "I don't know the IP address for the domain you requested, but here's the IPaddress for a name server that knows more than I do."

It can return an error message because the requested domain name is invalid or does notexist.When you type a URL into your browser, the browser's first step is to convert the domain name andhost name into an IP address so that the browser can go request a Web page from the machine atthat IP address (see How Web Servers Work for details on the whole process). To do this conversion,the browser has a conversation with a name server.

When you set up your machine on the Internet, you (or the software that you installed to connect toyour ISP) had to tell your machine what name server it should use for converting domain names to IPaddresses. On some systems, the DNS is dynamically fed to the machine when you connect to theISP, and on other machines it is hard-wired. If you are working on a Windows 95/98/ME machine,you can view your current name server with the command WINIPCFG.EXE(IPCONFIG for Windows2000/XP). On a UNIX machine, type nslookup along with your machine name. Any program on yourmachine that needs to talk to a name server to resolve a domain name knows what name server totalk to because it can get the IP address of your machine's name server from the operating system .

The browser therefore contacts its name server and says, "I need for you to convert a domain nameto an IP address for me." For example, if you type "www.howstuffworks.com" into your browser, thebrowser needs to convert that URL into an IP address. The browser will hand"www.howstuffworks.com" to its default name server and ask it to convert it.

The name server may already know the IP address for www.howstuffworks.com. That would be thecase if another request to resolve www.howstuffworks.com came in recently (name servers cache IPaddresses to speed things up). In that case, the name server can return the IP address immediately.Let's assume, however, that the name server has to start from scratch.

A name server would start its search for an IP address by contacting one of the root name servers .The root servers know the IP address for all of the name servers that handle the top-level domains.Your name server would ask the root for www.howstuffworks.com, and the root would say(assuming no caching), "I don't know the IP address for www.howstuffworks.com, but here's the IPaddress for the COM name server." Obviously, these root servers are vital to this whole process, so:
http://computer.howstuffworks.com/web-page.htmhttp://computer.howstuffworks.com/web-page.htmhttp://computer.howstuffworks.com/web-page.htmhttp://computer.howstuffworks.com/web-server.htmhttp://computer.howstuffworks.com/web-server.htmhttp://computer.howstuffworks.com/web-server.htmhttp://computer.howstuffworks.com/operating-system.htmhttp://computer.howstuffworks.com/operating-system.htmhttp://computer.howstuffworks.com/operating-system.htmhttp://computer.howstuffworks.com/cache.htmhttp://computer.howstuffworks.com/cache.htmhttp://computer.howstuffworks.com/cache.htmhttp://computer.howstuffworks.com/cache.htmhttp://computer.howstuffworks.com/operating-system.htmhttp://computer.howstuffworks.com/web-server.htmhttp://computer.howstuffworks.com/web-page.htm


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There are many of them scattered all over the planet.Every name server has a list of all of the known root servers. It contacts the first root serverin the list, and if that doesn't work it contacts the next one in the list, and so on.

The formatting is a little odd, but basically it shows you that the list contains the actual IP addresses

of 13 different root servers.The root server knows the IP addresses of the name servers handling the several hundred top-leveldomains. It returns to your name server the IP address for a name server for the COM domain. Yourname server then sends a query to the COM name server asking it if it knows the IP address forwww.howstuffworks.com. The name server for the COM domain knows the IP addresses for thename servers handling the HOWSTUFFWORKS.COM domain, so it returns those. Your name serverthen contacts the name server for HOWSTUFFWORKS.COM and asks if it knows the IP address forwww.howstuffworks.com. It does, so it returns the IP address to your name server, which returns itto the browser, which can then contact the server for www.howstuffworks.com to get a Web page.

One of the keys to making this work is redundancy . There are multiple name servers at every level,so if one fails, there are others to handle the requests. There are, for example, three differentmachines running name servers for HOWSTUFFWORKS.COM requests. All three would have to fail forthere to be a problem.

The other key is caching . Once a name server resolves a request, it caches all of the IP addresses itreceives. Once it has made a request to a root server for any COM domain, it knows the IP addressfor a name server handling the COM domain, so it doesn't have to bug the root servers again for thatinformation. Name servers can do this for every request, and this caching helps to keep things frombogging down.

Name servers do not cache forever, though. The caching has a component, called the Time To Live (TTL), that controls how long a server will cache a piece of information. When the server receives anIP address, it receives the TTL with it. The name server will cache the IP address for that period of

time (ranging from minutes to days) and then discard it. The TTL allows changes in name servers topropagate. Not all name servers respect the TTL they receive, however. When HowStuffWorksmoved its machines over to new servers, it took three weeks for the transition to propagatethroughout the Web. We put a little tag that said "new server" in the upper left corner of the homepage so people could tell whether they were seeing the new or the old server during the transition.

Creating a New Domain NameWhen someone wants to create a new domain, he or she has to do two things:

Find a name server for the domain name to live on. Register the domain name.

Technically, there does not need to be a machine in the domain -- there just needs to be a name

server that can handle the requests for the domain name.

There are two ways to get a name server for a domain:

You can create and administer it yourself.You can pay an ISP or hosting company to handle it for you.

Most larger companies have their own domain name servers. Most smaller companies pay someone.

The history of HowStuffWorks is typical. When howstuffworks.com was first created, it began as aparked domain . This domain lived with a company called www.webhosting.com . Webhosting.commaintained the name server and also maintained a machine that created the single "underconstruction" page for the domain.

To create a domain, you fill out a form with a company that does domain name registration(examples: register.com , verio.com , networksolutions.com ). They create an "under construction
http://computer.howstuffworks.com/cache.htmhttp://computer.howstuffworks.com/cache.htmhttp://computer.howstuffworks.com/cache.htmhttp://computer.howstuffworks.com/'framed.htm?parent=dns.htm&url=http://www.webhosting.com%27http://computer.howstuffworks.com/'framed.htm?parent=dns.htm&url=http://www.webhosting.com%27http://computer.howstuffworks.com/'framed.htm?parent=dns.htm&url=http://www.webhosting.com%27http://computer.howstuffworks.com/'framed.htm?parent=dns.htm&url=http://www.register.com%27http://computer.howstuffworks.com/'framed.htm?parent=dns.htm&url=http://www.register.com%27http://computer.howstuffworks.com/'framed.htm?parent=dns.htm&url=http://www.register.com%27http://computer.howstuffworks.com/'framed.htm?parent=dns.htm&url=http://www.verio.com%27http://computer.howstuffworks.com/'framed.htm?parent=dns.htm&url=http://www.verio.com%27http://computer.howstuffworks.com/'framed.htm?parent=dns.htm&url=http://www.verio.com%27http://computer.howstuffworks.com/'framed.htm?parent=dns.htm&url=http://www.networksolutions.com%27http://computer.howstuffworks.com/'framed.htm?parent=dns.htm&url=http://www.networksolutions.com%27http://computer.howstuffworks.com/'framed.htm?parent=dns.htm&url=http://www.networksolutions.com%27http://computer.howstuffworks.com/'framed.htm?parent=dns.htm&url=http://www.networksolutions.com%27http://computer.howstuffworks.com/'framed.htm?parent=dns.htm&url=http://www.verio.com%27http://computer.howstuffworks.com/'framed.htm?parent=dns.htm&url=http://www.register.com%27http://computer.howstuffworks.com/'framed.htm?parent=dns.htm&url=http://www.webhosting.com%27http://computer.howstuffworks.com/cache.htm


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page," create an entry in their name server, and submit the form's data into the whois database.Twice a day, the COM, ORG, NET, etc. name servers get updates with the newest IP addressinformation. At that point, a domain exists and people can go see the "under construction" page.

HowStuffWorks then started publishing content under the domain www.howstuffworks.com. We setup a hosting account with Tabnet (now part of Verio, Inc.), and Tabnet ran the DNS for

HowStuffWorks as well as the machine that hosted the HowStuffWorks Web pages. This type of machine is called a virtual Web hosting machine and is capable of hosting multiple domainssimultaneously. Five-hundred or so different domains all shared the same processor.

As HowStuffWorks became more popular, it outgrew the virtual hosting machine and needed its ownserver. At that point, we started maintaining our own machines dedicated to HowStuffWorks, andbegan administering our own DNS. We currently have four servers:

AUTH-NS1.HOWSTUFFWORKS.COM 70.42.150.19AUTH-NS2.HOWSTUFFWORKS.COM 70.42.150.20AUTH-NS3.HOWSTUFFWORKS.COM 70.42.251.19AUTH-NS4.HOWSTUFFWORKS.COM 70.42.251.20

Our primary DNS is auth-ns1.howstuffworks.com . Any changes we make to it propagateautomatically to the secondary, which is also maintained by our ISP.

All of these machines run name server software called BIND. BIND knows about all of the machines inour domain through a text file on the main server that looks like this:

@ NS auth-ns1.howstuffworks.com.@ NS auth-ns2.howstuffworks.com.@ MX 10 mail

mail A 209.170.137.42

vip1 A 216.183.103.150

www CNAME vip1

Decoding this file from the top, you can see that:

The first two lines point to the primary and secondary name servers .The next line is called the MX record . When you send e-mail to anyone at

howstuffworks.com, the piece of software sending the e-mail contacts the name server to get theMX record so it knows where the SMTP server for HowStuffWorks is (see How E-mail Works fordetails). Many larger systems have multiple machines handling incoming e-mail, and thereforemultiple MX records.

The next line points to the machine that will handle a request to mail.howstuffworks.com .

The next line points to the IP address that will handle a request to oak.howstuffworks.com .The next line points to the IP address that will handle a request to howstuffworks.com (nohost name).You can see from this file that there are several physical machines at separate IP addresses that makeup the HowStuffWorks server infrastructure. There are aliases for hosts like mail and www. There canbe aliases for anything. For example, there could be an entry in this file forscoobydoo.howstuffworks.com , and it could point to the physical machine called walnut. Therecould be an alias for yahoo.howstuffworks.com , and it could point to yahoo. There really is no limitto it. We could also create multiple name servers and segment our domain.

As you can see from this description, DNS is a rather amazing distributed database. It handles billionsof requests for billions of names every day through a network of millions of name servers

administered by millions of people. Every time you send an e-mail message or view a URL, you are
http://howstuffworks.com/framed.htm?parent=dns.htm&url=http://www.networksolutions.com/cgi-bin/whois/whoishttp://howstuffworks.com/framed.htm?parent=dns.htm&url=http://www.networksolutions.com/cgi-bin/whois/whoishttp://howstuffworks.com/framed.htm?parent=dns.htm&url=http://www.networksolutions.com/cgi-bin/whois/whoishttp://computer.howstuffworks.com/'framed.htm?parent=dns.htm&url=http://www.isc.org/products/BIND/%27http://computer.howstuffworks.com/'framed.htm?parent=dns.htm&url=http://www.isc.org/products/BIND/%27http://computer.howstuffworks.com/'framed.htm?parent=dns.htm&url=http://www.isc.org/products/BIND/%27http://computer.howstuffworks.com/'email.htm'http://computer.howstuffworks.com/'email.htm'http://computer.howstuffworks.com/'email.htm'http://computer.howstuffworks.com/'email.htm'http://computer.howstuffworks.com/'framed.htm?parent=dns.htm&url=http://www.isc.org/products/BIND/%27http://howstuffworks.com/framed.htm?parent=dns.htm&url=http://www.networksolutions.com/cgi-bin/whois/whois


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making requests to multiple name servers scattered all over the globe. What's amazing is that theprocess is usually completely invisible and extremely reliable!

ipconfig is a commmand line utility available on all versions of Microsoft Windows starting withWindows NT. ipconfig is designed to be run from the Windows command prompt. This utility allowsyou to get the IP address information of a Windows computer. It also allows some control over activeTCP/IP connections. ipconfig is an alternative to the older 'winipcfg' utility.
http://compnetworking.about.com/cs/basictcpip/g/bldef_tcpip.htmhttp://compnetworking.about.com/cs/basictcpip/g/bldef_tcpip.htmhttp://compnetworking.about.com/cs/basictcpip/g/bldef_tcpip.htm


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DEEL 2Ipconfig UsageFrom the command prompt, type 'ipconfig' to run the utility with default options. The output of the

default command contains the IP address, network mask and gateway for all physical and virtualnetwork adapters . Ipconfig' supports several command line options as described below. Thecommand "ipconfig /?" displays the set of available options.

ipconfig /allThis option displays the same IP addressing information for each adapter as the default option.Additionally, it displays DNS and WINS settings for each adapter.

ipconfig /releaseThis option terminates any active TCP/IP connections on all network adapters and releases those IPaddresses for use by other applications. 'ipconfig /release" can be used with specific Windows

connection names. In this case, the command will affect only the specified connections and not all.The command accepts either full connection names or wildcard names. Examples:ipconfig /release "Local Area Connection 1"ipconfig /release *Local*

ipconfig /renewThis option re-establishes TCP/IP connections on all network adapters. As with the release option,ipconfig /renew takes an optional connection name specifier.Both /renew and /release options only work on clients configured for dynamic (DHCP) addressing.Note: The remaining options below are only available on Windows 2000 and newer versions of

Windows.ipconfig /showclassid, ipconfig /setclassid These options manage DHCP class identifiers. DHCP classes can be defined by administrators on aDHCP server to apply different network settings to different types of clients. This is an advancedfeature of DHCP typically used in business networks, not home networks.

ipconfig /displaydns, ipconfig /flushdnsThese options access a local DNS cache that Windows maintains. The /displaydns option prints thecontents of the cache, and the /flushdns option erases the contents.This DNS cache contains a list of remote server names and the IP addresses (if any) they correspond

to. Entries in this cache come from DNS lookups that happen when attempting to visit Web sites,named FTP servers, and other remote hosts. Windows uses this cache to improve the performance of Internet Explorer and other Web-based applications.

In home networking, these DNS options are sometimes useful for advanced troubleshooting. If theinformation in your DNS cache becomes corrupted or outdated, you could face difficulty accessingcertain sites on the Internet. Consider these two scenarios:

The IP address of a Web site, email server or other server changes (rare occurence). The name

and address of this site normally stay in your cache for 24 hours after your last visit. You mayneed to clear your cache to access the server sooner.
http://compnetworking.about.com/od/hardwarenetworkgear/g/bldef_adapter.htmhttp://compnetworking.about.com/od/hardwarenetworkgear/g/bldef_adapter.htmhttp://compnetworking.about.com/cs/domainnamesystem/g/bldef_dns.htmhttp://compnetworking.about.com/cs/domainnamesystem/g/bldef_dns.htmhttp://compnetworking.about.com/cs/domainnamesystem/g/bldef_dns.htmhttp://compnetworking.about.com/cs/windowsnetworkin1/g/bldef_wins.htmhttp://compnetworking.about.com/cs/windowsnetworkin1/g/bldef_wins.htmhttp://compnetworking.about.com/cs/windowsnetworkin1/g/bldef_wins.htmhttp://compnetworking.about.com/cs/protocolsdhcp/g/bldef_dhcp.htmhttp://compnetworking.about.com/cs/protocolsdhcp/g/bldef_dhcp.htmhttp://compnetworking.about.com/cs/protocolsdhcp/g/bldef_dhcp.htmhttp://compnetworking.about.com/cs/protocolsdhcp/g/bldef_dhcp.htmhttp://compnetworking.about.com/cs/windowsnetworkin1/g/bldef_wins.htmhttp://compnetworking.about.com/cs/domainnamesystem/g/bldef_dns.htmhttp://compnetworking.about.com/od/hardwarenetworkgear/g/bldef_adapter.htm


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A Web site or other server was offline when you last visited it (hopefully a rare occurence) butsince has come back online. The cache will normally keep a record that the server is offline for 5minutes afer your last visit. You may need to clear your cache to access the server sooner.

ipconfig /registerdnsSimilar to the above options, this option updates DNS settings on the Windows computer. Instead of merely accessing the local DNS cache, however, this option initiates communication with both theDNS server (and the DHCP server) to re-register with them.

This option is useful in troubleshooting problems involving connection with the Internet serviceprovider, such as failure to obtain a dynamic IP address or failure to connect to the ISP DNS server.

Like the /release and /renew options, /registerdns optionally takes the name(s) of specific adaptersto update. If no name parameter is specified, /registerdns updates all adapters.


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DEEL 3

How DHCP Technology Works

Updated: March 28, 2003

How DHCP WorksDHCP provides an automated way to distribute and update IP addresses and other configuration

information on a network. A DHCP server provides this information to a DHCP client through the

exchange of a series of messages, known as the DHCP conversation or the DHCP transaction. If the

DHCP server and DHCP clients are located on different subnets, a DHCP relay agent is used to

facilitate the conversation.

NoteIt is necessary to have an understanding of basic TCP/IP concepts, including working knowledge of

subnets before you can have a full understanding of DHCP. For more information about TCP/IP, see

TCP/IP Technical Reference .

In this sectionDHCP Architecture

DHCP Protocols DHCP Processes and Interactions

DHCP Architecture

The DHCP architecture consists of DHCP clients, DHCP servers, and DHCP relay agents on a network.

The clients interact with servers using DHCP messages in a DHCP conversation to obtain and renew IP

address leases.
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DHCP Client Functionality

A DHCP client is any network-enabled device that supports the ability to communicate with a DHCP

server in compliance with RFC 2131, for the purpose of obtaining dynamic leased IP configuration

and related optional information.

DHCP provides support for client computers running any of the following Microsoft operating

systems:

Windows NT version 4.0

Windows 2000

Windows XP

Windows Server 2003

Windows 98

Windows Millennium Edition

Automatic IP Configuration

DHCP supports Automatic Private IP Addressing (APIPA), which enables computers running

Windows 2000, Windows XP, and Windows Server 2003 to configure an IP address and subnet mask

if a DHCP server is unavailable at system startup and the Automatic private IP address Alternate

Configuration setting is selected. This feature is useful for clients on small private networks, such as a

small-business office or a home office.

The DHCP Client service on a computer running Windows XP and Windows Server 2003 uses the

following process to auto-configure the client:

1. The DHCP client attempts to locate a DHCP server and obtain an IP address and

configuration.

2. If a DHCP server cannot be found or does not respond after one minute, the DHCP client

checks the settings on the Alternate Configuration tab of the properties of the TCP/IP

protocol.

If Automatic private IP address is selected, the DHCP client auto-configures its IP address

and subnet mask by using a selected address from the Microsoft-reserved Class B network,

169.254.0.0, with the subnet mask 255.255.0.0. The DHCP client tests for an address conflict

to ensure that the IP address is not in use on the network. If a conflict is found, the client

selects another IP address. The client retries auto-configuration up to 10 times.

If User Configured is selected, the DHCP client configures a static IP address configuration.

The DHCP client tests for an address conflict to ensure that the IP address is not already in


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use on the network. If a conflict is found, the DHCP client indicates the error condition to the

user.

3. When the DHCP client succeeds in self-selecting an address, it configures its network

interface with the IP address. The client then continues to check for a DHCP server in the

background every five minutes. If a DHCP server responds, the DHCP client abandons its self-

selected IP address and uses the address offered by the DHCP server (and any other DHCP

option information that the server provides) to update its IP configuration settings.

If the DHCP client obtained a lease from a DHCP server on a previous occasion, and the lease is still

valid (not expired) at system startup, the client tries to renew its lease. If, during the renewal

attempt, the client fails to locate any DHCP server, it attempts to ping the default gateway listed in

the lease, and proceeds in one of the following ways:

If the ping is successful, the DHCP client assumes that it is still located on the same network

where it obtained its current lease, and continues to use the lease as long as the lease is still

valid. By default the client then attempts, in the background, to renew its lease when 50

percent of its assigned lease time has expired.

If the ping fails, the DHCP client assumes that it has been moved to a network where a DHCP

server is not available. The client then auto-configures its IP address by using the settings on

the Alternate Configuration tab. When the client is auto-configured, it attempts to locate a

DHCP server and obtain a lease every five minutes.

Local Storage

Windows Server 2003 DHCP supports local storage, which allows clients to store DHCP information

on their own hard disks. Local storage is useful because it enables the client to store its last leased IP

address, so that when the client starts it first attempts to renew the lease of its previous IP address.

Local storage also enables a client to be shut down and restarted and it will use its previously leased

address and configuration, even if the DHCP server is unreachable or offline at the time that the

client computer is restarted.

DHCP Server Responsibilities

The DHCP servers maintain scopes, reservations, and options as set by the administrator.

Scopes

A scope must be properly defined and activated before DHCP clients can use the DHCP server for

automatic TCP/IP configuration. A DHCP scope is an administrative collection of IP addresses and

TCP/IP configuration parameters that are available for lease to DHCP clients of a specific subnet. Thenetwork administrator creates a scope for each subnet.


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A scope has the following properties:

A scope name, assigned when the scope is created.

A range of possible IP addresses from which to include or exclude addresses used in DHCP

lease offers.

A unique subnet mask, which determines the network ID for an IP address in the scope.

Lease duration values.

Each DHCP scope can have a single continuous range of IP addresses. To use several address ranges

within a single scope you must first define the entire address range for the scope, and then set

exclusion ranges.

Lease Durations

When a scope is created, the lease duration is set to eight days by default. However there are

situations when the administrator might want to change the lease duration. The following are

examples of adjusting the lease duration due to individual network consideration:

An organization has a large number of IP addresses available and configurations that rarely

change. The administrator increases the lease duration to reduce the frequency of lease

renewal exchanges between clients and the DHCP server. Because the DHCP clients are

renewing their leases less frequently, DHCP-related network traffic is reduced.

A limited number of IP addresses are available and client configurations change frequently or

clients move often in or out of the network. The administrator reduces the lease duration.

This increases the rate at which unused addresses are returned to the available address pool

for reassignment.

For example, consider the ratio between connected computers and available IP addresses. If 40

computers share 254 available addresses, the demand for reusing addresses is low. A long lease time,

such as a few months, might be appropriate in such a situation. However, if 230 computers must

share the same address pool, demand for available addresses is greater, and a shorter lease time, forexample a few days, is more appropriate.

Note

Although it is possible to configure a client with infinite lease duration, use infinite lease

durations with caution. Even relatively stable environments have a certain amount of client

turnover. At a minimum, computers might be added and removed, moved from one office to

another, or network adapters might be replaced. If a client with an infinite lease is removedfrom the network without releasing its lease, the DHCP server is not notified, and the IP


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Superscopes

A superscope allows a DHCP server to provide leases from more than one scope to clients on a single

physical subnet. Before you can create a superscope, you must use the DHCP Microsoft Management

Console (MMC) snap-in to define at least one of the scopes to be included in the superscope. Scopes

added to a superscope are called member scopes . Superscopes can resolve DHCP Server service

issues in several different ways; these issues include situations in which:

Support is needed for DHCP clients on a single physical network segment such as a single

Ethernet LAN segment where multiple logical IP networks are used. When more than one

logical IP network is used on a physical network, these configurations are also known as

multinets . In a situation where multinets are used, clients might not be able to communicate

directly with each other, because the clients might be on different logical subnets, even if

they are on the same physical network segment. In this case, routing must be enabled to

allow the clients to communicate with each other. Also, a router or BOOTP/DHCP relay agent

must be configured on the subnet to allow DHCP messages to travel between the logical

subnets.

Support is needed for DHCP clients that are in a multinet located on the other side of BOOTP

relay agents.

Clients need to be migrated to a new scope.

Interactions between Client and Server

DHCP servers and DHCP clients communicate through a series of DHCP messages. To obtain a lease,

the DHCP client initiates a conversation with a DHCP server using a series of these DHCP messages.

DHCP Messages

The following list includes the eight types of messages that can be sent between DHCP clients and

servers. For more information about the structure and specifics of each of these packets, see DHCP

Message Format later in this section.


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DHCPDiscover

Broadcast by a DHCP client when it first attempts to connect to the network. The DHCPDiscover

message requests IP address information from a DHCP server.

DHCPOffer

Broadcast by each DHCP server that receives the client DHCPDiscover message and has an IP address

configuration to offer to the client. The DHCPOffer message contains an unleased IP address and

additional TCP/IP configuration information, such as the subnet mask and default gateway. More

than one DHCP server can respond with a DHCPOffer message. The client accepts the best offer,

which for a Windows DHCP client is the first DHCPOffer message that it receives.

DHCPRequestBroadcast by a DHCP client after it selects a DHCPOffer. The DHCPRequest message contains the IP

address from the DHCPOffer that it selected. If the client is renewing or rebinding to a previous lease,

this packet might be unicast directly to the server.

DHCPAck

Broadcast by a DHCP server to a DHCP client acknowledging the DHCPRequest message. At this time,

the server also forwards any options. Upon receipt of the DHCPAck, the client can use the leased IPaddress to participate in the TCP/IP network and complete its system startup. This message is

typically broadcast, because the DHCP client does not officially have an IP address that it can use at

this point. If the DHCPAck is in response to a DHCPInform, then the message is unicast directly to the

host that sent the DHCPInform message.

DHCPNack

Broadcast by a DHCP server to a DHCP client denying the clients DHCPRequest message. This might

occur if the requested address is incorrect because the client moved to a new subnet or because the

DHCP clients lease has expired and cannot be renewed.


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DHCPDecline

Broadcast by a DHCP client to a DHCP server, informing the server that the offered IP address is

declined because it appears to be in use by another computer.

DHCPRelease

Sent by a DHCP client to a DHCP server, relinquishing an IP address and canceling the remaining

lease. This is unicast to the server that provided the lease.

DHCPInform

Sent from a DHCP client to a DHCP server, asking only for additional local configuration parameters;

the client already has a configured IP address. This message type is also used by DHCP servers

running Windows Server 2003 to detect unauthorized DHCP servers.

DHCP Lease Process

A DHCP-enabled client obtains a lease for an IP address from a DHCP server. Before the lease expires,

the DHCP client must renew the lease or obtain a new lease. Leases are retained in the DHCP server

database for a period of time after expiration. By default, this grace period is four hours and cleanup

occurs once an hour for a DHCP server running Windows Server 2003. This protects a clients lease in

case the client and server are in different time zones, the internal clocks of the client and servercomputers are not synchronized, or the client is off the network when the lease expires.

Obtaining a New Lease

A DHCP client initiates a conversation with a DHCP server when it is seeking a new lease, renewing a

lease, rebinding, or restarting. The DHCP conversation consists of a series of DHCP messages passed

between the DHCP client and DHCP servers. The following figure shows an overview of this process

when the DHCP server and DHCP client are on the same subnet.

DHCP Lease Process Overview


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1. The DHCP client requests an IP address by broadcasting a DHCPDiscover message to the local

subnet.

2. The client is offered an address when a DHCP server responds with a DHCPOffer message

containing an IP address and configuration information for lease to the client. If no DHCP

server responds to the client request, the client sends DHCPDiscover messages at intervals of

0, 4, 8, 16, and 32 seconds, plus a random interval of between -1 second and 1 second. If

there is no response from a DHCP server after one minute, the client can proceed in one of

two ways:

If the client is using the Automatic Private IP Addressing (APIPA) alternate

configuration, the client self-configures an IP address for its interface.

If the client does not support alternate configuration, such as APIPA, or if IP auto-

configuration has been disabled, the client network initialization fails.

In both cases, the client begins a new cycle of DHCPDiscover messages in the background

every five minutes, using the same intervals as before (0, 4, 8, 16, and 32 seconds), until it

receives a DHCPOffer message from a DHCP server.

3. The client indicates acceptance of the offer by selecting the offered address and

broadcasting a DHCPRequest message in response.

4. The client is assigned the address and the DHCP server broadcasts a DHCPAck message inresponse, finalizing the terms of the lease.

When the client receives acknowledgment, it configures its TCP/IP properties by using the DHCP

option information in the reply, and completes its initialization of TCP/IP.

In rare cases, a DHCP server might return a negative acknowledgment to the client. This can happen

if a client requests an invalid or duplicate address. If a client receives a negative acknowledgment

(DHCPNack), the client must begin the entire lease process again.


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lease. The client then begins the lease process again, and is offered the next available address in the

scope.

Note

ARP requests do not traverse routers. Clients use ARP requests rather than pings (ICMP Echo

messages) because pings require the sender to have an IP address.

Server Conflict Detection

If your network includes older DHCP clients that do not perform conflict detection themselves, you

can enable conflict detection on the DHCP server. By default, the Windows Server 2003 DHCP Server

service does not perform any conflict detection.

To detect conflicts, the DHCP server pings (sends an ICMP Echo message to) an IP address beforeoffering that address to clients in a new lease. The DHCP server only pings addresses that have not

been successfully and previously leased. If a client requests a lease on an IP address that it already

had or is requesting a renewal, the DHCP server does not ping the IP address.

If conflict detection is enabled, an administrator-defined number of pings are sent. The server waits 1

second for a reply. Because the time required for a client to obtain a lease is equal to the number of

pings used, choose this value carefully because it directly impacts the overall performance of the

server. In general, one ping is sufficient.

If a response to the ping is received, a conflict is registered and that address is not offered to clients

requesting a lease from the server. The DHCP server then attaches a BAD_ADDRESS value to that IP

address in the scope. The DHCP server then tries to lease the next available address. If the duplicate

address is removed from the network, the BAD_ADDRESS value attached to the IP address can be

deleted from the scopes list of active leases, and then the address returns to the pool. Addresses are

marked as BAD_ADDRESS for the length of the lease for which the scope is configured. If the

BAD_ADDRESS entry is not manually removed, it will automatically be removed after a period of time

equal to the lease time for the scope.

Note

In general, use server conflict detection only as a troubleshooting aid when you suspect that

duplicate IP addresses are in use on your network. Each additional conflict detection attempt

adds to the time needed to negotiate leases for DHCP clients.


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DHCP Options

DHCP options are additional configuration parameters that a DHCP server assigns to clients. Options

can also be used for DHCP communication between the server computer and client computers.

The most specific options take precedence over the least specific options. This simplifies DHCPmanagement and allows a flexible administration that can range from per-server default settings to

common settings for a specific subnet and individualized client settings when needed for special

circumstances. In most cases, the option values are specified in the Options dialog box on the DHCP

server, scope, or reservation.

DHCP options can be configured for specific values and enabled for assignment and distribution to

DHCP clients based on:

Server options. These options apply globally for all scopes and classes defined at each DHCP

server and any clients that it services. Configured server option values always apply unless

they are overridden by options assigned to other scope, class, or client reservation.

Scope options. These options apply to any clients that obtain a lease within that particular

scope. Configured scope option values always apply to all computers obtaining a lease in a

given scope unless they are overridden by options assigned to class or client reservation.

Class options. These options apply to any clients that specify that particular DHCP Class ID

value when obtaining a scope lease. Configured class option values always apply to allcomputers configured as members in a specified DHCP option class unless they are

overridden by options assigned to a client reservation.

Reserved client options. These options apply only to the client corresponding to the

reservation. Reserved client option values override all other server, scope, or class assigned

option values.

Options are typically applied at each DHCP server at the server or scope level. To precisely manage or

customize option settings for a group or class of computers, specify either a user or vendor class

assignment that overrides the broader server or scope option defaults.

For special requirements, such as clients with special functions, assign options for specific reserved

clients.

Options can also be used to separate and distribute appropriate options for clients with similar or

special configuration needs. For example, DHCP clients on the same floor of a building can be

configured with the same DHCP Class ID value to assign them membership in the same option class.

You can then distribute additional or varied option data to that class during the lease process,

overriding any scope or globally provided default options.


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Note

Statically configured values on a client override any DHCP options of any type or level.

Many options are predefined on a DHCP server running Windows Server 2003. Other standard DHCP

options can be added as needed to support any other DHCP client software that recognizes or

requires the use of these additional options. The DHCP Server service running on Windows

Server 2003 supports all options defined in RFC 2132, although most DHCP clients use or support

only a small subset of the available RFC-specified options.

The following table contains a list of default DHCP options requested by DHCP clients running

Windows Server 2003 and Windows XP. For a complete reference of DHCP options, see DHCP Tools

and Settings .

Default DHCP Options

Option Code Option Name

1 Subnet mask

3 Router

6 DNS servers

15 DNS domain name

44 WINS/NBNS servers

46 WINS/NetBT node type

47 NetBIOS scope ID

51 Lease time

58 Renewal (T1) time value

59 Rebinding (T2) time value
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31 Perform router discovery

33 Static route

43 Vendor-specific information

249 Classless static routes

DHCP Option Parameters

DHCP servers can be configured to provide optional data that fully configures TCP/IP on a client.

Some of the most common DHCP option types configured and distributed by the DHCP server during

the lease process include parameters for the default gateway, DNS, and WINS.

Clients can be configured with:

Information options. You can explicitly configure these options and any associated values

provided to clients.

Protocol options. You can implicitly configure these options used by the DHCP Server service

based on server and scope property settings.

You can use the DHCP snap-in to configure these properties and set them for an entire scope or for a

single, reserved client scope.

Information Options

The following table lists the most common types of DHCP information options that can be configured

for DHCP clients. These options can be enabled and configured for each scope that you configure on

a DHCP server. Depending on your network infrastructure, some of these options can be configured

as server options, such as DNS domain name.

Common Information Options

Code Description

3 Router

6 DNS server


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15 DNS domain name

44 WINS/NBNS servers

Clients can request these DHCP options, and can use the values to set their TCP/IP configurations for

the duration of the lease.

Protocol Options

The following table shows protocol options that DHCP clients can be configured to use when

communicating with a DHCP server to obtain or renew a lease.

Common Protocol Options

Code Description

51 Lease time

53 DHCP message type

55 Special option type used to communicate a parameter request list to the DHCP server

58 Renewal time value (T1)

59 Rebind time value (T2)

The values provided to clients for lease time, T1, and T2 are taken from the scope settings on the

DHCP server. The value provided for DHCP message type is automatically set depending on which

packet of the DHCP conversation is being sent.

Option Classes

Option classes allow quick introduction of custom applications for enterprise networks. DHCP option

classes provide a way to easily configure network clients with the parameters necessary to meet the

special requirements of custom applications. Equipment from multiple vendors on a network can

also use different option code numbers for different functions. The options used to support vendor

classes the vendor class identifier and the vendor-specific option are defined in the Internet

DHCP options standard reference, RFC 2132.


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Windows Server 2003 includes two types of option classes: vendor-defined and user-defined. These

classes can be configured on your servers to offer specialized client support in the following ways:

Add and configure vendor-defined classes for managing DHCP options assigned to clients

identified by vendor type.

Add and configure user-defined classes for managing DHCP options assigned to clients that

need a similar DHCP option configuration.

After options classes are defined on a DHCP server, scopes on the server can be configured to assign

options for specific user-defined and vendor-defined option classes.

Vendor Classes

Vendor- defined option classes can be used by DHCP clients to identify the clients vendor type and

configuration when obtaining a lease from the DHCP server. The client can include the vendor class

ID option (option code 60) when it requests or selects a lease from a DHCP server to identify its

vendor class during the lease process.

The vendor class identifier information is a string of character data interpreted by the DHCP servers.

Vendors can choose to define specific vendor class identifiers to convey particular configuration or

other identification information about a client. For example, the identifier might encode the clients

hardware or software configuration. Most vendor types are derived from standard reserved

hardware and operating system-type abbreviation codes listed in RFC 1700.

When vendor options are specified, the server performs the following additional steps to provide a

lease to the client:

The server verifies that the vendor class identified by the client request is a recognized class

defined on the server.

If the vendor class is recognized, the server checks to see if any additional DHCP options are

configured for this class in the active scope.

If the vendor class is not recognized, the server ignores the vendor class identified in the

client request, and returns options allocated to the default vendor class (which includes all

DHCP Standard options).

If the scope contains options configured specifically for use with clients in this vendor-

defined class, the server returns those options using the vendor-specific option type (option

code 43) as part of its acknowledgment message.

In most cases, the default vendor class the DHCP Standard option class provides a default

vendor class for any Windows DHCP clients or other DHCP clients that do not specify a vendor class

ID. In some cases, you might define additional vendor classes for other DHCP clients, such as printers


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or some types of UNIX clients. When you add other vendor classes for these purposes, make sure

that the vendor class identifier you use to configure the class at the server matches the identifier

used by clients for your third-party vendor.

User ClassesUser classes allow DHCP clients to differentiate themselves by specifying what type of client they are,

such as desktop or server computer. For computers running Windows Server 2003, Windows XP, and

Windows 2000, you can define specific user class identifiers to convey information about a clients

software configuration, its physical location in a building, or about its user preferences. For example,

an identifier can specify that DHCP clients are members of a user class called 2nd floor, West,

which has need for a specific set of router, DNS, and WINS server settings. An administrator can then

configure the DHCP server to include different option values depending on the user class of client

receiving the lease.

Windows Server 2003 user classes can be used as follows:

DHCP client computers can include the DHCP user class option when sending DHCP request

messages to the DHCP server. This can specifically identify the client as part of a user class on

the server.

DHCP servers running the Windows 2000 Server or Windows Server 2003 DHCP Server

service can recognize and interpret the DHCP user class option from clients and provide

additional options (or a mo dified set of DHCP options) based on the clients user class

identity.

For example, shorter leases can be assigned to wireless clients. Or perhaps a particular set of clients

might need a specific set of routes, a specific DNS server, or a specific default gateway.

Note

If user classes are not specified, default settings, such as server options or scope options, are

assigned.

A user class can be either a default or custom user class. Microsoft provides three default user

classes, as described in the following table.

Default User Classes Provided by Windows DHCP

Class Type Class ID String Description


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Default UserClass

(Unspecified) This class is typically used by most DHCP clients. Clients that areincluded in this class:

DHCP clients that cannot be configured with a user

class or a user class ID. This is true for most Windows-based DHCP clients prior to Windows 2000.

Clients running Windows Server 2003, Windows XP, or

Windows 2000 configured with a class ID unknown to the

DHCP server.

Clients that do not otherwise specify a user class ID.

Default

Routing andRemoteAccess class

RRAS.Microsoft This class is used by the Windows 2000 Server or Windows

Server 2003 DHCP Server service to classify clients making aPPP-type connection through a remote access server. Typically,this class includes most dial-up networking clients that useDHCP to obtain a lease, including remote access clients thatcannot be configured with a Routing and Remote Access userclass or a Routing and Remote Access user class ID.

See DHCP and Routing and Remote Access later in this topicfor details about the interaction between a Routing and RemoteAccess server and a DHCP server and how DHCP servers identify

remote access clients.

DefaultBOOTP class

BOOTP This class is used by the Windows 2000 Server or WindowsServer 2003 DHCP Server service to classify any clientsrecognized as BOOTP clients.

Use the Microsoft default user classes to isolate specific configuration details for clients with special

needs, such as older clients or clients that use BOOTP or Routing and Remote Access. For example,

you might want to include and assign special BOOTP option types (such as option codes 66 and 67)

for clients that are BOOTP type, or shorten the lease time for remote access clients.

You can also add and configure custom user classes for use by DHCP clients running Windows 2000,

Windows XP, and Windows Server 2003. For a custom user class to work properly, the client must

use the same custom identifier when requesting options as was used when the class was defined on

the DHCP server

The user class option field permits only one ASCII text string to be used for identifying clients. This

means each client computer can be identified only as a member of a single user class by the DHCP

server. You can use additional user classes to make new hybrids from your other user classes to


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accommodate clients that need configuration for multiple user classes. For example, if you have two

user classes, one called mobile with short lease times assigned and another called engineer with

an option assigned to configure a high-performance server for its clients, you can make a new hybrid

user class called mobile -engineer that contain s both special option value settings.

MADCAP and Multicast DHCP

Multicast Address Dynamic Client Allocation Protocol (MADCAP) is modeled after the DHCP standard.

MADCAP assists in simplifying and automating configuration of multicast groups on your network,

but it is not required for the operation of multicast groups or for the DHCP Server service. Multicast

scopes provide only multicast address configuration and do not support or use other DHCP-

assignable options.

Multicast scopes configured on the DHCP server define ranges of IP multicast addresses. Similar toallocating unicast IP addresses, IP multicast addresses are allocated to MADCAP clients. A MADCAP

address is configured separately from a primary IP address. Computers that use either static or

dynamic IP configuration through a DHCP server can be MADCAP clients.

In Windows Server 2003, the DHCP Server service supports both DHCP and MADCAP, although these

services function separately. Clients of one do not depend on the use or configuration of the other.

Clients that do not support the MADCAP service or are unable to contact and obtain multicast

configuration from a MADCAP server can be configured in other ways so that they participate ineither permanent or temporary multicast groups on the network.

In all TCP/IP networks, each computer requires a unique primary unicast IP address for each network

interface. You must assign this required primary unicast IP address before you can configure a

computer to support and use secondary IP addresses such as multicast IP addresses.

DHCP Protocols

In Windows Server 2003, the DHCP Server service includes support for the Dynamic HostConfiguration Protocol (DHCP), the Multicast Address Dynamic Client Allocation Protocol (MADCAP),

and the Bootstrap Protocol (BOOTP).

DHCP

DHCP servers communicate with DHCP clients by using a series of DHCP messages. The format of

DHCP messages is based on the message format used with the BOOTP protocol.

RFC 2131 defines the format for each message sent between a DHCP client and a DHCP server. The

following table shows the possible fields in the DHCP messages.


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DHCP Message Fields

FieldName

FriendlyName

FieldLength(Octets) Description

op MessageType

1 Message type

htype HardwareAddressType

1 Hardware address type. Defined athttp://www.iana.org/assignments/arp-parameters

hlen HardwareAddressLength

1 Hardware address length in octets

hops Hops 1 Value is set to zero by DHCP clients. Optionally used tocount the number of relay agents that forwarded themessage.

xid TransactionID

4 A random number used to associate messages andresponses between a client and a server.

secs Seconds 2 Seconds elapsed since client began address acquisition orrenewal process.

flags Flags 2 Flags set by client. The Broadcast flag is set if the clientcannot receive unicast IP datagrams (for example, before itis configured with an IP address).

ciaddr Client IPAddress

4 This field is only filled in if the client has an IP address andcan respond to ARP requests.

yiaddr Your IPAddress

4 Address given to the DHCP client by the DHCP server

siaddr DHCP Server

IP Address

4 IP address of the server that is offering a lease


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giaddr Gateway IPAddress

4 DHCP relay agent IP address

chaddr Client

HardwareAddress

16 Client hardware address

sname Server HostName

64 Optional server host name. Not used in WindowsServer 2003

file Boot FileName

128 The name of the file containing the boot image for a BOOTPclient

options Options variable Optional parameters field. In the DHCP protocol packet,each option begins with a single octet tag, which holds theoption code, and a second octet, which describes the optiondata length, in bytes. For a complete list of the DHCPoptions available by default on a DHCP server running onWindows Server 2003, see DHCP Tools and Settings .

For a complete view of how these fields are used in each DHCP message, see RFC 2131 or use a

network monitoring tool, such as Netmon, to view the DHCP messages.

MADCAP

Windows Server 2003 includes a Multicast Address Dynamic Client Allocation Protocol (MADCAP)

Server service to support dynamic assignment and configuration of IP multicast addresses on TCP/IP-

based networks.

Whereas DHCP unicast scopes provide client configurations by allocating ranges of IP addresses for

point-to-point communication between two networked computers, multicast scopes provide ranges

for multicast IP addresses. These addresses are reserved for multicast operation using directedtransmission from one point to multiple points.

A multicast address is shared by many computers. A group of TCP/IP computers can use a single

multicast IP address to send directed communication to all computers with which they share the use

of the group address. An IP datagram that is sent to the multicast address is forwarded to all

members of that multicast group.
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Dynamic Membership

Multicast addresses support dynamic membership, allowing individual computers to join or leave the

multicast group at any time. The size of the group is not limited, and computers can be members of

multiple groups. In addition, any computer that uses TCP/IP can send datagrams to any multicast

group.

Multicast Address Ranges

You can permanently reserve multicast group addresses or temporarily assign and use them. A

permanent group is made by permanently reserving a multicast IP address (224.0.0.0 to

239.255.255.255) with the Internet Assigned Numbers Authority (IANA). The reserved address then

becomes a well-known address, indicating a specific multicast group that exists regardless of

whether group member computers are present on the network. Any multicast IP address that is not

permanently reserved with the IANA can then be used dynamically to assign and form temporary

multicast groups. These temporary groups can exist as long as one or more computers on the

network are configured with the groups address and actively share in its use.

BOOTP

Bootstrap Protocol (BOOTP) is a computer configuration protocol developed before DHCP. DHCP

improves on BOOTP and resolves specific limitations that BOOTP had as a computer configuration

service. RFC 951 defines BOOTP.

Whereas BOOTP configures diskless workstations with limited boot capabilities, DHCP configures

networked computers, that have local hard drives and full boot capabilities.

Likewise, although both BOOTP and DHCP allocate IP addresses to clients during startup, they use

different methods of allocation. BOOTP typically provides fixed allocation of a single IP address for

each client, permanently reserving this address in the BOOTP server database. DHCP typically

provides dynamic, leased allocation of available IP addresses, reserving each DHCP client address

temporarily in the DHCP database.

Because of the relationship between BOOTP and DHCP, both protocols share some defining

characteristics. BOOTP and DHCP use nearly identical request messages and reply messages. Both

protocols enclose each protocol message in a single User Datagram Protocol (UDP) datagram of 576

bytes. Message headers are the same for both BOOTP and DHCP, except for the final message header

field that carries optional data. For BOOTP, this optional field is called the vendor-specific area and is

limited to 64 bytes. For DHCP, this optional field is called the options field and is at least 312 bytes

long.


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Both BOOTP and DHCP use the same reserved protocol ports for sending and receiving messages

between servers and clients. Both BOOTP and DHCP servers use UDP port 67 to listen for and receive

client request messages. BOOTP and DHCP clients typically reserve UDP port 68 for accepting

message replies from either a BOOTP server or DHCP server.

Because DHCP and BOOTP messages use nearly identical format types and packet structures, and use

the same well-known service ports, BOOTP or DHCP relay agent programs usually treat BOOTP and

DHCP messages as the same message type and do not differentiate between them.

BOOTP clients do not rebind or renew configuration with the BOOTP server except when the system

restarts, whereas DHCP clients do not require a system restart to rebind or renew configuration with

the DHCP server. Instead, clients automatically enter the rebinding state at defined intervals to

renew their leased address allocation with the DHCP server. This process occurs in the background

and is transparent to the user.

BOOTP uses a two-phase bootstrap configuration process in which clients contact BOOTP servers to

perform address determination and boot file name selection, and clients also contact Trivial File

Transfer Protocol (TFTP) servers to perform file transfer of their boot image. DHCP uses a single-

phase boot configuration process whereby a DHCP client negotiates with a DHCP server to determine

its IP address and obtain any other initial configuration details it needs for network operation.

Because BOOTP clients contact TFTP servers to perform file transfer of their boot image andWindows Server 2003 does not provide a TFTP file service, you need a third-party TFTP server to

support BOOTP clients that must boot from an image file (usually diskless workstations). You also

need to configure your DHCP server to provide supported BOOTP/DHCP options.

DHCP Options Supported for BOOTP Clients

To obtain other options, BOOTP clients must specify DHCP option code 55 (the Options Request List

parameter) in the BOOTP request. BOOTP clients that do not specify option 55 can still retrieve the

options listed in the following table from DHCP servers running Windows NT Server 4.0 or later, if

they are configured on the server.

DHCP Options for BOOTP Clients

Code Option Name

1 Subnet Mask


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3 Router

4 Time Server

5 Name Server

9 LPR Server

12 Computer Name

15 Domain Name

17 Root Path

42 NTP Servers

44 WINS Server

45 NetBIOS over TCP/IP Datagram Distribution Server

46 NetBIOS over TCP/IP Node Type

47 NetBIOS over TCP/IP Scope

48 X Window System Font Server

49 X Window System Display Manager

69 SMTP Server

70 POP3 Server

DHCP servers running Windows Server 2003 return the options in the order listed above and return

as many options as can fit in a single datagram response. For more information about individual

DHCP options, see DHCP Tools and Settings .

Note
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When configuring client reservations for use with BOOTP clients, remember that DHCP

options can apply equally to DHCP and BOOTP clients.

BOOTP Table

Each record in the BOOTP table has three fields of information that is returned to the BOOTP client:

Boot Image . Identifies the ge neric file name (such as unix) of the requested boot file, based

on the BOOTP clients hardware type.

File Name . Identifies the full path of the boot file (such as /etc/vmunix) that the BOOTP

server returns to the client by using TFTP.

File Server . Identifies the name of the TFTP server used to store the boot file.

To add entries in the BOOTP table, use the DHCP snap-in.

DHCP Processes and Interactions

In Windows Server 2003, the DHCP Server service interacts with several other services, including the

Active Directory directory service, DNS, and the Routing and Remote Access service.

Detecting Unauthorized DHCP Servers

An unauthorized DHCP server on a network can cause a variety of problems, such as the leasing of

incorrect IP addresses and options. To protect against this type of problem, when a DHCP server

running Windows 2000 or Windows Server 2003 starts on the network, it first attempts to determine

if it is authorized to service clients. There are different methods used depending on how the network

is configured.

Unauthorized Domain Member DHCP Servers

A domain member DHCP server queries Active Directory. The DHCP server compares its IP address

and server name to the list of authorized DHCP servers. If either the server name or IP address is

found on the list of authorized DHCP servers, the server is authorized as a DHCP server. If no match is

found, the server is not authorized in Active Directory, the server does not respond to DHCP traffic,and a system event is logged.

Note

This process of authorizing DHCP servers is useful for only DHCP servers running

Windows 2000 or Windows Server 2003. This process cannot be used for DHCP servers

running Windows NT Server 4.0, or servers running non-Windows-based DHCP Server

services. Only a member of the Enterprise Admins group can authorize or unauthorize a

DHCP server in Active Directory.


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Unauthorized Workgroup DHCP Servers

A Windows Server 2003 workgroup member DHCP server uses the following process to detect other

DHCP servers currently running on the reachable network and to determine if it is authorized to

provide service.

1. When the DHCP Server service starts, it sends a DHCPInform request message to the

reachable network, using the local limited broadcast address (255.255.255.255), to locate

other DHCP servers on the network.

This message includes several vendor-specific option types that are known and supported by

other DHCP servers running Windows Server 2003. These other DHCP servers will respond

with a DHCPAck containing information indicating if they are authorized domain member or

workgroup member servers.

2. When queried, other DHCP servers running Windows 2000 and Windows Server 2003 reply

with DHCPAck messages to acknowledge and answer with workgroup or domain

membership information.

3. If an Active Directory domain member DHCP server is found, then the workgroup member

server determines that it is not authorized and does not service clients. If other workgroup

servers are found, the workgroup member server determines that it is authorized to service

clients, and begins service. It then performs the check again at one-hour intervals.

DHCP and DNS

Domain Name System (DNS) servers provide name resolution for network clients. DNS resolves a fully

qualified domain name (FQDN) to its corresponding IP address.

Although DHCP provides a powerful mechanism for automatically configuring client IP addresses,

prior to Windows 2000, the DHCP Server service did not notify DNS to update the DNS records on

behalf of the client. Specifically, DHCP did not map the client name to an IP address and did not

update IP address-to-name mappings using DNS dynamic update.

Without a way for DHCP to interact with DNS, the information maintained by DNS for a DHCP client

might be incorrect. For example, a client can acquire its IP address from a DHCP server, but the DNS

records might not reflect the IP address acquired nor provide a mapping from the new IP address to

the FQDN.

DNS Dynamic Updates

In Windows 2000 and Windows Server 2003, DHCP servers and clients can register record updates if

the DNS server supports DNS dynamic updates. In Windows 2000 Server and Windows Server 2003,

the DNS service supports DNS dynamic updates.


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A DHCP server running Windows Server 2003 can register with a DNS server and update pointer

(PTR) and address (A) resource records on behalf of its DHCP-enabled clients by using the DNS

dynamic update protocol.

The ability to register A and PTR resource records lets a DHCP server act as a DNS registration proxyfor clients using Windows NT 4.0, Windows 98, or Windows Millennium Edition, and possibly other

clients that are not able to register the updates on their own, as shown in the following figure.

DHCP Server Performing DNS Dynamic Update on Behalf of DHCP Client

DHCP clients running Windows 2000, Windows XP, and Windows Server 2003 interact with DNS

differently than DHCP clients running earlier versions of Windows. DHCP clients running Windows XP,

Windows 2000, or Windows Server 2003 typically update their own dynamic forward lookup names,as shown in the following figure.

DHCP Client and DHCP Server Performing DNS Dynamic Update

An additional DHCP option code (option code 81) enables the return of a clients FQDN to the DHCP

server. If implemented, the DHCP server can dynamically update an individual comp uters resource

records on a DNS server by using the DNS dynamic update protocol.

For more information about DNS dynamic updates, see DNS Technical Reference .
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