mcgraw-hill©the mcgraw-hill companies, inc., 2000 chapter 4 network layer

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Chapter 4

Network Layer

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Understand the position of the network layer in the Internet Understand the position of the network layer in the Internet model.model.

Understand the rationale for the existence of the network layer.Understand the rationale for the existence of the network layer.

Understand the concept of host-to-host delivery.Understand the concept of host-to-host delivery.

Understand the duties of the network layer: packetizing, Understand the duties of the network layer: packetizing, addressing, and routing. addressing, and routing.

After reading this chapter, the reader should After reading this chapter, the reader should be able to:be able to:

OOBJECTIVESBJECTIVES

Know which upper-layer protocol can use the services of IP.Know which upper-layer protocol can use the services of IP.

Understand the network layer protocol, IP, used in the Internet.Understand the network layer protocol, IP, used in the Internet.

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INTRODUCTIONINTRODUCTIONINTRODUCTIONINTRODUCTION

4.14.1

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Figure 4-1

Network layer in the Internet model

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Figure 4-2

Duties of the network layer

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ADDRESSINGADDRESSINGADDRESSINGADDRESSING

4.24.2

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The network addresses The network addresses must be unique.must be unique.

Note:Note:

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Technical Focus:Technical Focus: Binary Notation of IP AddressesBinary Notation of IP Addresses

An IP address is stored as a binary number in the computer. A 4-part dotted-decimal address can be converted to binary if we replace each part by its binary equivalent. The following shows the IP address 10.34.234.8 in dotted-decimal and binary notation:

10.34.224.8

00001010 00100010 11100000 00001000

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Technical Focus:Technical Focus: Address SpaceAddress Space

A protocol such as IP that defines addresses has an address space. An address space is the total number of addresses available to the protocol. If a protocol uses N bits to define an address, the address space is 2N because each bit can have two different values (0 and 1); N bits can have 2N values.

The Internet uses 32-bit addresses, which means that the address space is 232 or 4,294,967,296 (more than 4 billion). This means that theoretically, if there were no restrictions, more than 4 billion devices could be connected to the Internet.

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Figure 4-3

Two levels of hierarchy

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When using two levels of When using two levels of address hierarchy, the address hierarchy, the

common part is referred to common part is referred to as the as the netidnetid or or prefixprefix and and

the variable part is referred the variable part is referred to as the to as the hostidhostid, or , or suffixsuffix..

Note:Note:

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Figure 4-4

Three levels of hierarchy

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With three levels of address With three levels of address hierarchy, the common part hierarchy, the common part is referred to as the is referred to as the netidnetid. .

The part common to all The part common to all computers connected to the computers connected to the same subnet is referred to same subnet is referred to

as the subnetid. The unique as the subnetid. The unique part is referred to as the part is referred to as the

hostidhostid..

Note:Note:

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A packet traveling from the A packet traveling from the source to the destination source to the destination

needs at least four needs at least four addresses: source and addresses: source and

destination port numbers destination port numbers and source and destination and source and destination

IP addresses.IP addresses.

Note:Note:

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Figure 4-5

DNS example

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ROUTINGROUTINGROUTINGROUTING

4.34.3

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Figure 4-6

Example of portion of the Internet

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Figure 4-7

Routing decisions

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Figure 4-8

Routing table for router R1

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Figure 4-9

Routing tables

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Routing tables can be based Routing tables can be based on next-hop routing.on next-hop routing.

Note:Note:

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Routing tables for the Internet Routing tables for the Internet are usually network-specific are usually network-specific

rather than host-specific. The rather than host-specific. The router routes the packet to the router routes the packet to the

final network; it is then final network; it is then broadcast to reach the final broadcast to reach the final

destination. destination.

Network-based routing reduces Network-based routing reduces the size of routing tables.the size of routing tables.

Note:Note:

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Figure 4-10

Autonomous systems

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Routing in the Internet is Routing in the Internet is hierarchical. Delivery is first hierarchical. Delivery is first

made to the autonomous made to the autonomous system, then to the system, then to the

network, and finally to the network, and finally to the host. Hierarchical routing host. Hierarchical routing

reduces the size of the reduces the size of the routing tables. routing tables.

Note:Note:

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PACKETIZING:PACKETIZING:IP PROTOCOLIP PROTOCOL

PACKETIZING:PACKETIZING:IP PROTOCOLIP PROTOCOL

4.44.4

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Figure 4-11

Position of IP in Internet model

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Figure 4-12

IP datagram format

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Technical Focus:Technical Focus: Fields in a DatagramFields in a Datagram

The fields in a datagram are as follows:The fields in a datagram are as follows:

VersionVersion

Header lengthHeader length

Differentiated serviceDifferentiated service

Total lengthTotal length

Identification, flags, and fragmentation offsetIdentification, flags, and fragmentation offset

Time to liveTime to live

ChecksumChecksum

ProtocolProtocol

Source addressSource address

Destination addressDestination address

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FRAGMENTATIONFRAGMENTATIONFRAGMENTATIONFRAGMENTATION

4.54.5