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Network Layer:19.1 Internetworks19.2 Addressing

Classful, Classless addressing, NAT

19.3 Routing

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Figure 19.2 Links in an internetwork

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Figure 19.3 Network layer in an internetwork

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Figure 19.4 Network layer at the source

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IP Packet Headerbits

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Figure 19.5 Network layer at a router

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Example Example

A router inside the organization receives the same packet with destination address 190.240.33.91. Show how it finds the subnetwork address to route the packet.

SolutionSolution

The router follows three steps:1. The router must know the mask. Assume it is /19.2. The router applies the mask to the address, 190.240.33.91. The subnet

address is 190.240.32.0. 3. The router looks in its routing table to find how to route the packet to

this destination.

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Next-hop address

Interface

191.240.11.0191.240.12.0190.240.32.0

f1f2f3

191.240.12.0

190.240.32.0

191.240.11.0

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Figure 19.6 Network layer at the destination

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Figure 19.7 Switching

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Figure 19.7 Switching

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Figure 19.7 Switching

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Figure 19.8 Datagram approach

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Switching at the network layer in the Internet is done using the datagram

approach to packet switching.

NoteNote::

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Communication at the network layer in the Internet is connectionless.

NoteNote::

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19.3 Routing19.3 Routing

Routing Techniques

Static Versus Dynamic Routing

Routing Table for Classful Addressing

Routing Table for Classless Addressing

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Routing Techniques

Next-hop routing Network-specific routing Host-specific routing Default routing

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Figure 19.28 Next-hop routing

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Figure 19.29 Network-specific routing

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Figure 19.30 Host-specific routing

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Figure 19.31 Default routing

Network address 0.0.0.0

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Static and Dynamic Routing Tables

Routes manually entered

By administrator

If Internet changes, table is updated manually

Good for small internet, experiments, troubleshooting

Routes periodically updated

By protocols like RIP, OSPF, BGP (ch21)

If Internet changes, protocols update tables.

Good for big internet such as the Internet.

Static Dynamic

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Figure 19.32 Classful addressing routing table

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Example 10Example 10

Using the table in Figure 19.32, the router receives a packet for destination 192.16.7.1. For each row, the mask is applied to the destination address until a match with the destination address is found.

In this example, the router sends the packet through interface m0 (host specific).

192.16.7.1 11000000 00010000 00000111 0000000111111111 11111111 11111111 1111111111000000 00010000 00000111 00000001

/32

192.16.7.1

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Example 11Example 11

Using the table in Figure 19.32, the router receives a packet for destination 193.14.5.22. For each row, the mask is applied to the destination address until a match with the next-hop address is found.

In this example, the router sends the packet through interface m2 (network specific).

193.14.5.22 11000001 00001110 00000101 0001011011111111 11111111 11111111 0000000011000001 00001110 00000101 00000000

/24

193.14.5.0

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Example 12Example 12

Using the table in Figure 19.32, the router receives a packet for destination 200.34.12.34. For each row, the mask is applied to the destination address, but no match is found.

In this example, the router sends the packet through the default interface m0.

200.34.12.34 11001000 00100010 00001100 0010001000000000 00000000 00000000 0000000000000000 00000000 00000000 00000000

/0

0.0.0.0

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Figure 19.32 Classless addressing routing table

Mask Destination address

Next-hop address

Interface

/24/24…/24/25/25

190.100.0.0190.100.1.0

…190.100.63.0190.100.64.0190.100.64.12

8

118.45.23.8

202.45.9.3…

84.78.8.12146.11.10.

614.1.10.5

m0m1…m3m0m2

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An example of address allocation and distribution by an ISP

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No Hierarchical routingMask Destination

addressNext-hop address

Interface

/24/24/24/25/25/24/24/24/25/25/24/24/24/25/25

ABCDEFGHIJKLMOPQ

-BCBBBBBBCCCCCCC

-M0M1M0M0M0M0M0M0M1M1M1M1M1M1M1

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Hierarchical routing (2-level)Mask Destination

addressNext-hop address

Interface

/24/24/24/25/25/24/24

ABC

Region 2Region 3Region 4Region 5

-BCBBCC

-M0M1M0M0M1M1

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Geographic routing

A U.S. backbone

A European backbone

Leased transatlantic lines

Leased lines to Asia

Regional networkNational network

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Summary

Classful addressing (obsolete) Wasteful address archetecture Network boundaries are fixed at 8, 16

and 24 bits (class A, B, and C) Classless addressing

Efficient address architecture Network boundaries may occur at any bit (/12, /16,/19,/24, etc)

Best Current Practice

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Summary

NAT allows private network to use a set of

private addresses internally, and global addresses externally.

Routing Static / Dynamic Classful / Classless routing Hierarchical / Geographical routing

Alleviates IP depletion

Prevent immense routing table sizes

Used for the Internet