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Network Layer (Part 3)

Computer Networks

Tutun JuhanaTelecommunication EngineeringSchool of Electrical Engineering & Informatics

Institut Teknologi Bandung

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DELIVERY AND FORWARDINGOF IP PACKETS

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Direct Delivery

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Indirect Delivery

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Forwarding

Forwarding means to place the packet in its

route to its destination

to deliver the packet to the next hop (which can be

the final destination or the intermediate connectingdevice)

Two kinds of forwarding

1. The destination addressbased forwarding (when

IP is used as a connectionless protocol)2. Label-basedforwarding (when the IP is used as a

connection-oriented protocol)

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FORWARDING BASED ONDESTINATION ADDRESS

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Next-Hop Method

The routing table holds only the address of

the next hop instead of information about

the complete route

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Host-Specific Method

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Default Method

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FORWARDING WITHCLASSFUL ADDRESSING

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Forwarding without Subnetting

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Example 6.2

Router R1 in Figure 6.8 receives a packet with destination address

192.16.7.14. Show how the packet is forwarded

Solution

The destination address in binary is 11000000 00010000 00000111

00001110. A copy of the address is shifted 28 bits to the right. Theresult is 00000000 00000000 00000000 00001100 or

12. The destination network is class C. The network address is

extracted by masking off the leftmost 24 bits of the destination

address; the result is 192.16.7.0. The table for Class C is searched.

The network address is found in the first row. The next-hop address111.15.17.32. and the interface m0 are passed to ARP

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Forwarding with Subnetting

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Forwarding with Classless Addressing

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Address Aggregation

In classful addressing, there is only oneentry in the routing table for each site

outside the organization

In classless addressing, it is likely that thenumber of routing table entries will

increaseThe increased size of the table

results in an increase in the amount oftime needed to search the tableto

alleviate the problem, the idea of address

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Longest Mask Matching

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The routing table is sorted

from the longest mask to the

shortest mask

Packet arrives withdest. add.140.24.7.200

By longest mask

matching

correctly routed

to Organization 4

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STRUCTURE OF A ROUTER

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Components

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performs the functions of the network layer

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