chapter 4, slide: 1 cs 372 – introduction to computer networks* friday july 23, 2010...

Chapter 4, slide: 1

CS 372 – introduction to computer networks*Friday July 23, 2010

Announcements:

Midterms are graded. Lab 4 is posted.

Acknowledgement: slides drawn heavily from Kurose & Ross

* Based in part on slides by Bechir Hamdaoui and Paul D. Paulson.

Internet addresses

A key aspect of a virtual network is a single, uniform address format

Can't use hardware addresses because different technologies have different address formats

Can't use addresses that are local to a network because multiple networks might use the same addresses internally

Address format must be independent of any particular hardware address format

Chapter 4, slide: 2

IP address notation

IP address is just a 32-bit number It’s the same internally, regardless of its external

representation In decimal form, the range is [0 … 4294967296] Some are reserved In hexadecimal form, the range is [00000000 …

FFFFFFFF]• divides naturally into 4 2-hexdigit groups• e.g.: 80 FD 28 1C• each group represents one byte (octet)• Binary form is 10000000 11111101 00101000 00011100

For convenience, use dotted decimal notation e.g.: 128.253.40.28 (= 80.FD.28.1C) range is [0.0.0.0 … 255.255.255.255]

Chapter 4, slide: 3

Chapter 4, slide: 4

IP Addressing: introduction IP address: 32-bit

identifier for host, router interface

interface: connection between host/router and physical link multiple interfaces per

router one interface per host one IP address per

interface

223.1.1.1

223.1.1.2

223.1.1.3

223.1.1.4 223.1.2.9

223.1.2.2

223.1.2.1

223.1.3.2223.1.3.1

223.1.3.27

223.1.1.1 = 11011111 00000001 00000001 00000001

223 1 11

Chapter 4, slide: 5

Subnets IP address:

subnet part (higher bits) host part (lower bits)

What’s a subnet ? device interfaces with

same subnet part of IP address

can physically reach each other without intervening router

network consisting of 3 subnets

223.1.1.1

223.1.1.2

223.1.1.3

223.1.1.4 223.1.2.9

223.1.2.2

223.1.2.1

223.1.3.2223.1.3.1

223.1.3.27

subnet11001000 00010111 00010000 00000000

subnetpart

hostpart

200.23.16.0/23

Chapter 4, slide: 6

Subnets 223.1.1.0/24223.1.2.0/24

223.1.3.0/24

Recipe To determine the

subnets, detach each interface from its host or router, creating islands of isolated networks. Each isolated network is called a subnet. Subnet mask: /24

Chapter 4, slide: 7

SubnetsHow many? 223.1.1.1

223.1.1.3

223.1.1.4

223.1.2.2223.1.2.1

223.1.2.6

223.1.3.2223.1.3.1

223.1.3.27

223.1.1.2

223.1.7.0

223.1.7.1223.1.8.0223.1.8.1

223.1.9.1

223.1.9.2

Chapter 4, slide: 8

IP addresses: how to get one?

Q: How does host get IP address?

hard-coded by system admin in a file

DHCP: Dynamic Host Configuration Protocol: dynamically get IP address from as server when

joining the network IP address can be reused by other hosts if released Can renew IP addresses if stayed connected

Chapter 4, slide: 9

DHCP client-server scenario

223.1.1.1

223.1.1.2

223.1.1.3

223.1.1.4 223.1.2.9

223.1.2.2

223.1.2.1

223.1.3.2223.1.3.1

223.1.3.27

A

BE

DHCP server

arriving DHCP client needsaddress in thisnetwork

Chapter 4, slide: 10

IP addresses: how to get one?

Q: How does network get subnet part of IP addr?

A: gets allocated portion of its provider ISP’s address space

ISP's block 11001000 00010111 00010000 00000000 200.23.16.0/20

Organization 0 11001000 00010111 00010000 00000000 200.23.16.0/23 Organization 1 11001000 00010111 00010010 00000000 200.23.18.0/23 Organization 2 11001000 00010111 00010100 00000000 200.23.20.0/23 ... ….. …. ….

Organization 7 11001000 00010111 00011110 00000000 200.23.30.0/23


IP addressing: the last word...

Q: How does an ISP get block of addresses?

A: ICANN: Internet Corporation for Assigned

Names and Numbers allocates addresses manages DNS assigns domain names, resolves disputes

IP address format

Each IP address is divided into a prefix and a suffix

Prefix identifies the network and the type of network to which a host computer is attached

Suffix identifies a host computer within that network

Usually includes indicator for number of bits used for prefix

Address format enables efficient routingChapter 4, slide: 12

IP address hierarchy

Every network in a TCP/IP internet is assigned a unique network number

Each host on a specific network is assigned a host number or host address that is unique within that network

Host's IP address is the combination of the network number (prefix) and host address (suffix)


IP address assignment

Network numbers (prefixes) are unique Host addresses (suffixes) may be

duplicated on different networks The combination of network number prefix

and host address suffix is unique in the entire internet


IP address assignment

Assignment of network numbers must be coordinated globally

Assignment of host addresses can be managed locally


IP address design

IP-v4 designers chose 32-bit addresses Allocate some bits for prefix, some for

suffix Large prefix, small suffix - many networks,

few hosts per network Small prefix, large suffix - few networks,

many hosts per network Because of the wide variety of

technologies, need to allow for both large and small networks


IP addressing (two types):Classful addressing: A, B, C

A: /8 B: /16 C: /24

CIDR: Classless InterDomain Routing network portion of address of arbitrary length address format: a.b.c.d/x, where x is # bits in network

portion of address

11001000 00010111 000100 00 00000000

networkpart

hostpart

200.23.16.0/22

(only 28 networks, but 224 hosts per network)

(216 networks, and 216 hosts per network)(224 networks, but only 28 hosts per

network)


IP address classes: Classful addressing

Multiple address formats that allow both large and small prefixes

Each format is called an address class The class of an address is identified by

first four bits The number of bits allocated for the

prefix is determined by the class



Class A, B and C are primary classes Used for ordinary host addressing

Class D is used for multicast, a limited form of broadcast Internet hosts join a multicast group Packets are delivered to all members of group Routers manage delivery of single packet from source to

all members of multicast group Used for multicast backbone

Class E is reserved


Determining IP class

Dotted decimal makes separating network address from host address easier

Look at first dotted decimal number, and use this table:


How many networks? Classful scheme does not yield equal number of

networks in each class class A:

First bit must be 0 7 remaining bits identify Class A net 27 (= 128) possible class A nets

• Minus a few that are reserved class B:

First 2 bits must be 10 14 remaining bits identify Class B net 214 (= 16384) possible class B nets

• Minus a few that are reserved class C:

First 3 bits must be 110 21 remaining bits identify Class C net 221 (= 2097152) possible class C nets

• Minus a few that are reservedChapter 4, slide: 23

Networks and hosts in each class

8

16

24


What if the form doesn't fit?

Large organizations may not be able to get as many addresses in the Internet as they need

Example - UPS needs addresses for millions of computers

Example – School needs 6000 hosts Too big for class C, too many wasted addresses

for class B


Possible solutions

Classless addressing allow division between prefix and suffix at any bit

boundary Sharing an IP address

Use one IP address for multiple hosts


Classless Addressing Example

128.193.47.25 dotted decimal80 C1 2F 19 hexadecimal1000 0000 1100 0001 0010 1111 0001 1001 binary

This can be “re-aligned” to use variable-size prefixes and suffixes.

Example: Suppose we want a 22-bit prefix and a 10-bit suffix


Example: 1000 0000 1100 0001 0010 1111 0001 1001 Logical AND with a "mask" using 22 bits for

prefix (netmask), 10 bits for suffix:1111 1111 1111 1111 1111 1100 0000 0000

… gives a prefix:1000 0000 1100 0001 0010 1100 0000 0000

The complement mask (hostmask)0000 0000 0000 0000 0000 0011 1111 1111

… gives a suffix:0000 0000 0000 0000 0000 0011 0001 1001


Example:1000 0000 1100 0001 0010 1100 0000 00000000 0000 0000 0000 0000 0011 0001 1001i.e., the “network number” is 80C12C00h, and the

“host number” within the network is 319h With 10 bits for suffix, 1024 host addresses

are available within the subnet (but 2 of these are reserved)

Address still looks almost the same in dotted decimal 128.193.47.25 / 22 Additional information is provided so addressing

can be handled by routers


CIDR

CIDR (Classless Inter-Domain Routing) address includes specification for number of bits to use for the netmask Example: host address 128.193.47.25/22 What is the netmask?

• 255.255.252.0 What is the network address?

• 128.193.44.0 What is the hostmask?

• 0.0.3.255 What is the host number?

• 0.0.3.25 = 319h = 793 (decimal)


CIDR

What is the netmask for /20 ? 255.255.240.0



How many hosts can be supported in /28 ? 24 - 2 = 14



ExampleSubnet 1

Subnet 2

Subnet 3

Three subnets All interfaces in all these

subnets are required to have prefix: 223.1.13/24

Subnet 1 is required to support 125 interfaces

Subnet 2 & 3 are each required to support 60 interfaces

Question: Provide 3 network addresses in the form: a.b.c.d/x

IP address assignment Select an address class for each network

depending on expected number of hosts Assign network numbers from appropriate classes Assign host suffixes to form internet addresses

for all hosts The lowest host number (0) is not used because

that complete address is the network address e.g., 192.168.5.0

The highest host number (depends on network type) is not used because that complete address is the “broadcast address” for the network e.g., 192.168.5.255


OSU IP addresses

Oregon State has a single Class B network: 128.193.0.0

All hosts at OSU have 128.193 prefix E.G.:

ns1.oregonstate.edu 128.193.0.10 ns2.oregonstate.edu 128.193.4.20

Suffix bytes are used to determine local network and host through subnetting

Individual host addresses assigned by system administrators may be static or dynamic assignment


IP address allocation

Addresses in the Internet are not used efficiently Less than 10% of possible addresses are

actually assigned Concerns about address space being

exhausted OSU is like most organizations, using

5,000-6,000 out of possible 216 (= 65,536) available addresses


Routing table information

In the routing table Destination stored as network address Next hop stored as IP address of router

Address mask defines how many bits of address are in prefix Prefix defines how much of address used to

identify network e.g., class B mask is 255.255.0.0

• In binary: 11111111111111110000000000000000


Example:

Routing table for


Consider 192.4.10.26:

Routing table for

doesn’t care what does with the packets Chapter 4, slide: 38

Address masks

To identify destination network, apply address mask to destination address and compare to network address in routing table

Use Boolean and if ((Mask[i] & D) == Dest[i])

forward to NextHop[i]


chapter 4, slide: 1 cs 372 – introduction to computer networks* friday july 23, 2010...

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