1

CSCI 233Internet Protocols

Class 3

Dave Roberts

FIRST…A LITTLE REVIEW

2

3

Internet Protocol Principles • Good Citizen Principle• Scarcest Internet Resource

4

Outline• Internet addresses• Mapping IP addresses to physical

addresses

INTERNET ADDRESSING

5

The Internet• Is it a physical or virtual network?• It’s a virtual network, defined by

protocols that run on hosts and routers.

• Internet protocols make the Internet look like a world-wide uniform network, although it encompasses many networks that are very different from each other.

6

Internet Addresses• Each host connection on the

Internet has a unique Internet address

• The addresses are designed to make forwarding of Internet packets simple

• An IP address has two parts: a prefix that identifies a network and a suffix that identifies a host on the network 7

Need for Control• To avoid conflicts in address use, some

sort of authority is needed• It makes sense to assign addresses in

blocks, not one at a time• ICANN (Internet Corporation for

Assigned names and Numbers) oversees IP address assignment

• Originally assigned in blocks of Class A, B and C addresses

8

Question• How many Internet addresses can one

host have?• As many as it has network adapters

9

IPv6 Addressing• Each address is 128 bits• Enough addresses for every

person on earth to have an internet with three times the addresses of the present Internet!

• 1024 addresses per square meter of the earth’s surface

10

IPv6 Address Assignments

11

IPv4 to IPv6 Transition

12

IPv6 Address Split

13

IP Addresses

14

Functions of Internet Addresses

• Provide a unique identification for a particular interface between a device and the network so that a datagram can be delivered to the correct recipient

• Enable a path to be found across the Internet to reach the recipient, a process called routing

15

16

IPv4 Address• 32-bit integer, unique for each

host on the network, used in all communication with the host

• <IP address> ::= <netid> <hostid>– Netid: identifier of a network– Hostid: identifier of a host on the

network

17

Dotted Decimal Notation

32-bit Internet address

10000000 00001010 00000010 00011110

Is written

128.10.2.30

18

Classes of IPv4 Addresses

“Classful” addresses—types A, B and C below

first 2 bits distinguish 3 primary classes

Design of these classes is for efficient routing

There have been other refinements—to discuss later

Class Determination Algorithm

19

Address Class Characteristics

20

Host Capacities

21

IP Address Split

22

Advantages of Classful Addressing

• Simplicity and clarity—addresses and their setup are very easy to understand

• Flexibility to accommodate different sizes of networks

• Ease of separating host address for routing

• Allows for reservation of some addresses for special purposes

23

Classless IPv4 Addressing• Temporary addressing scheme

that does away with class A, B, C addresses

• Network prefix can be any specified length

• Forwarding techniques expanded to account for this: called Classless Inter-Domain Routing (CIDR)

24

25

Special IP Addresses

ADDRESS RESOLUTION

26

IP and MAC Addresses• IP address is used to send datagrams

across the Internet—that is, between networks, through routers

• MAC address is used to deliver a frame of data within a single network

• We send a datagram across the Internet with only an IP address

• To deliver to a device at the destination network, a MAC address must be used

27

Local Delivery• The router at the destination

network has the job of delivering the packet to the appropriate host

• The router uses the local physical network to deliver to the local host

• The local physical (MAC) address must be used, not the IP address

28

Address Resolution• The process of determining the

physical address that corresponds to an IP address is called address resolution

• Address resolution must occur at every network the packet encounters in its journey across the Internet

29

Resolution by Direct Mapping

30

HA = f(IA)

Resolution by Dynamic Binding

• ARP broadcasts a request packet• Host who has IP address in packet

replies with physical address

31

ARP Caching• Broadcasting an inquiry is

expensive• Every host must have a cache of

recently acquired bindings• Results of ARP requests are

cached• Before sending request, the cache

is checked

32

ARP Cache Timeout• Responsibility for cache

correctness is with the host maintaining the cache

• Timeout value is set, and addresses from cache are not used if timeout value is exceeded

• ARP performance is sensitive to the value of the timeout

33

Soft State• ARP cache is an example of “soft

state”• Cache owner keeps record of

acquired results, avoids cost of future inquiries

• Cache is usually timed out to automatically remove stale values

34

ARP Refinements• Every ARP request has binding of

source IP and MAC addresses• Since request is broadcast, all

machines can extract sender’s IP to MAC address mapping and cache it

• Most computers broadcast a gratuitous ARP request when they start up in case their mapping has changed 35

IPv6 ARP• Describe IPv6 ARP• There isn’t any!!!

36

37

IPv4 ARP Message Format

Hardware type: 1 for EthernetProtocol type: 0800 for IP addressesXlen—length of physical and high-level addressesARP exchanges involve filling in missing addresses

IPv6 Neighbor Discovery• Neighbor is another computer on

the same network• NDP allows an IPv6 host to

discover all neighbors and routers upon startup

• Early binding avoids delays when packets are transmitted

38

39

Conventions• 1’s refer to “all”• 0’s refer to “this”• Hostid=0 address refers to this

network• Hostid=1’s broadcast to all hosts

– Directed broadcast—netid of a specific network

– Local broadcast—32 1’s—used at startup

Address Resolution• Physical addresses are used by

hardware devices that can communicate directly (ie, MAC addresses on a LAN)

• IP addresses create a massive virtual network

• Network layer sends datagrams across the virtual network

• Data link layer sends frames between physical devices

40

Need for Address Resolution

41

42

ARP• Each host has an Internet address,

Ia• Each host also has a physical

address, Pa • How to route packet to physical

address, given its Internet address?

• Two instances– Sending packets to routers, which

have physical addresses– Sending packets to hosts, which have

physical addresses

43

Direct Mapping Resolution• Choose a numbering scheme that

makes address resolution efficientPA = f(IA)

• If either P’s or I’s can be chosen, a correspondence can be established

• Alternatively, lists of P-A pairs can be stored

44

Dynamic Binding Resolution

ARP—Address Resolution Protocol• host A broadcasts packet with

address IB• Asks host B to respond with PB

• B recognizes the packet, responds with PB

• A receives response, uses PB to send to B

45

ARP Cache• Cache of recently-acquired

physical addresses is kept• ARP is used for packets after the

first in a transmission• ARP cache times out after an

interval• Example of “soft state”

46

ARP Refinements• Sender includes its own I to P

binding in every ARP broadcast, so that receiving site, and others, can update ARP caches

• Receivers update I to P binding in ARP cache before processing ARP packet

47

ARP Encapsulation

48

Internet Protocol Principles • Use the Good Citizen Principle to

limit the impact of resource shortages

• Conserve the scarcest Internet resource

• Use caching to avoid repeated inquiries

Summary• IP Addressing• IP Address Resolution

49