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This article explains the concepts of addressing and routing in internetworks, including classful and classless addressing, as well as the use of Network Address Translation (NAT). It also covers different routing techniques and their advantages.
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Network Layer: 19.1 Internetworks 19.2 Addressing Classful, Classless addressing, NAT 19.3 Routing
IP Packet Header bits
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. Solution • The router follows three steps: • The router must know the mask. Assume it is /19. • The router applies the mask to the address, 190.240.33.91. The subnet address is 190.240.32.0. • The router looks in its routing table to find how to route the packet to this destination.
191.240.11.0 190.240.32.0 191.240.12.0
Note: Switching at the network layer in the Internet is done using the datagram approach to packet switching.
Note: Communication at the network layer in the Internet is connectionless.
19.3 Routing Routing Techniques Static Versus Dynamic Routing Routing Table for Classful Addressing Routing Table for Classless Addressing
Next-hop routing Network-specific routing Host-specific routing Default routing Routing Techniques
Network address 0.0.0.0 Figure 19.31Default routing
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 and Dynamic Routing Tables Static Dynamic
Example 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). 11000000 00010000 00000111 00000001 11111111 11111111 11111111 11111111 11000000 00010000 00000111 00000001 192.16.7.1 /32 192.16.7.1
Example 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). 11000001 00001110 00000101 00010110 11111111 11111111 11111111 00000000 11000001 00001110 00000101 00000000 193.14.5.22 /24 193.14.5.0
Example 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. 11001000 00100010 00001100 00100010 00000000 00000000 0000000000000000 00000000 00000000 00000000 00000000 200.34.12.34 /0 0.0.0.0
Geographic routing A European backbone A U.S. backbone Leased lines to Asia Leased transatlantic lines National network Regional network
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) Summary Best Current Practice
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 Summary Alleviates IP depletion Used for the Internet Prevent immense routing table sizes
