Network Layer (Part 3)

Computer Networks

Tutun JuhanaTelecommunication EngineeringSchool of Electrical Engineering & InformaticsInstitut 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 connecting device)

• Two kinds of forwarding1. The destination address –based forwarding (when

IP is used as a connectionless protocol)

2. Label-based forwarding (when the IP is used as a connection-oriented protocol)

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FORWARDING BASED ON DESTINATION ADDRESS

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Forwarding Techniques

1. Next-Hop Method

2. Network-Specific Method

3. Host-Specific Method

4. Default Method

To make the size of the routing table manageable

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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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Network-Specific Method

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Host-Specific Method

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Default Method

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FORWARDING WITH CLASSFUL ADDRESSING

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Forwarding without Subnetting

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• Example 6.2Router 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. The result 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 address 111.15.17.32. 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 one entry in the routing table for each site outside the organization

• In classless addressing, it is likely that the number of routing table entries will increase The increased size of the table results in an increase in the amount of time needed to search the table to alleviate the problem, the idea of address aggregation was designed

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Longest Mask Matching

The routing table is sorted from the longest mask to the

shortest mask

Packet arrives with dest. 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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Componentsperforms the functions of the network layer

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Switching Fabrics

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