• The fast growth of Internet led to the near depletion of the available addresses.
• We have run out of class A and B addresses, and a class C block is too small for most midsize organizations.
• To overcome the problem of address depletion and give more organizations access to internet, classess addressing was designed and implemented.
• In this scheme, there are no classes, but the addresses are still granted in blocks.
• In classless addressing, when an entity, small or large, needs to be connected to Internet, it is granted a block or range of addresses.
• The size of the block (the number of addresses) varies based on the nature and size of the entry. For example, a household may be given only two addresses; a large organization may be given thousands of addresses. An ISP, may be given thousands or hundreds of thousands based on the number of customer it may serve.
• To simplify the handling of addresses, the Internet authorities impose three restrictions on classless address blocks:
1. The addresses in a block must be contiguous, one after the other.
2. The number of addresses in a block must be a power of 2 (1, 2, 4,8, …. ).
3. The first address must be evenly divisible by the number of address.
• For example, a block of addresses (both in binary and dotted decimal notation) granted to a small business that needs 16 addresses.
• As shown in figure certain restrictions are applied to this block. The addresses are contiguous. The number of addresses is a power of 2 (16=24), and the first address is divisible by 16. The first address, converted to decimal number, is 3,440,387,360, which whet divided by 16 results in 215,024,210.