2.1 OVERVIEW 27 ClassA 0 Network Host 14 Class B 10 Network Host 21 Class C 110 Network Host 28 Class D 1110 Multicast network Figure 2.2 Classful addresses [1]. addresses rather than being limited to 8,16,or 24 bits in length for the network part.This type of granularity provides an organization with more precise matches for IP address space requirements.The growth of forwarding table entries is also slowed by allowing address aggregation to occur at several levels within the heirarchy of the Internet's topology.Back- bone routers can now maintain the forwarding information at the level of the arbitrary aggregates of networks,instead of at the network level only. For example,consider the networks represented by the network numbers from 208.12.16/24 through 208.12.31/24(see Fig.2.3)and in arouter all these network addresses are reachable through the same service provider.The leftmost 20 bits of all the addresses in this range are the same (11010000 00001100 0001).Thus,these 16 networks can be aggregated into one 'supemetwork'represented by the 20-bit prefix,which in decimal notation gives 208.12.16/20.Indicating the prefix length is necessary in decimal notation, because the same value may be associated with prefixes of different lengths;for instance, 208.12.16/20(11010000000011000001*)is different from208.12.16/22(11010000 00001100000100*). Address aggregation does not reduce entries in the router's forwarding table for all cases. Consider the scenario where a customer owns the network 208.12.21/24 and changes its service provider,but does not want to renumber its network.Now,all the networks from 208.12.16/24 through 208.12.31/24 can be reached through the same service provider, except for the network 208.12.21/24 (see Fig.2.4).We cannot perform aggregation as before,and instead of only one entry,16 entries need to be stored in the forwarding table. One solution is aggregating in spite of the exception networks and additionally storing2.1 OVERVIEW 27 Figure 2.2 Classful addresses [1]. addresses rather than being limited to 8, 16, or 24 bits in length for the network part. This type of granularity provides an organization with more precise matches for IP address space requirements. The growth of forwarding table entries is also slowed by allowing address aggregation to occur at several levels within the heirarchy of the Internet’s topology. Back￾bone routers can now maintain the forwarding information at the level of the arbitrary aggregates of networks, instead of at the network level only. For example, consider the networks represented by the network numbers from 208.12.16/24 through 208.12.31/24 (see Fig. 2.3) and in a router all these network addresses are reachable through the same service provider. The leftmost 20 bits of all the addresses in this range are the same (11010000 00001100 0001). Thus, these 16 networks can be aggregated into one ‘supernetwork’ represented by the 20-bit prefix, which in decimal notation gives 208.12.16/20. Indicating the prefix length is necessary in decimal notation, because the same value may be associated with prefixes of different lengths; for instance, 208.12.16/20 (11010000 00001100 0001*) is different from 208.12.16/22 (11010000 00001100 000100*). Address aggregation does not reduce entries in the router’s forwarding table for all cases. Consider the scenario where a customer owns the network 208.12.21/24 and changes its service provider, but does not want to renumber its network. Now, all the networks from 208.12.16/24 through 208.12.31/24 can be reached through the same service provider, except for the network 208.12.21/24 (see Fig. 2.4). We cannot perform aggregation as before, and instead of only one entry, 16 entries need to be stored in the forwarding table. One solution is aggregating in spite of the exception networks and additionally storing