21.7 SUPERNET ADDRESSING

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In the Internet addressing scheme, it is likely that the class B addresses get exhausted faster than class A or class C addresses. To overcome this problem, the supernet addressing scheme is introduced. Instead of allocating a class B address, a site can be allocated a chunk of class C addresses. At the site, the physical networks can be allocated these class C addresses.

In supernet addressing, a site is allocated a block of class C addresses instead of one class B address. However, this results in a large routing table. To solve this problem, classless inter-domain routing technique is used.

In addition to making better use of address space, this supernet addressing scheme allows a hierarchy to be developed at a site. For instance, an Internet service provider (ISP) can be given a block of class C addresses that he can use to create a supernet with the networks of the ISP's subscribers.

Instead of one class B address, if a large number of class C addresses are given to a site, the routing table becomes very large. To solve this problem, the classless inter-domain routing (CIDR) technique is used, in which the routing table contains the entries in the format (network address, count)

where network address is the starting of the IP address block and count is the number of IP addresses in that block.

If each ISP is given a block of class C addresses with one entry in the routing table, the routing to that ISP becomes very easy.

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