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The Internet's explosive growth eventually required the more efficient use of the IP address space available. Variable Length Subnet Masking is a technique used to allow more efficient assignment of IP addresses. Originally Internet addresses were carved up into small, medium and large size blocks of contiguous addresses based on the vaues of the first four bits in the first octet of the IP address. These were often referred to as classful addresses. By carving classful address blocks into smaller classless blocks, you waste fewer addresses. The process of carving out smaller blocks from the larger blocks was called subnetting.

Many organization's networks started very small and were assigned class C addresses. A class C address range contains 256 addresses. Soon, these organizations grew and so did their networks. Networks that needed to expand beyond their original class C range used a technique called supernetting to allow them to turn two contiguous IP address blocks into one network.

SUBNETTING and SUPERNETTING

Originally the entire range of IP addresses were carved up into small, medium and large chunks of addresses. Networking equipment figured which addresses were all part of a network by looking at the first four bits of the address. There were five classes of addresses used. Subnetting is the process of borrowing bits from the host portion of an address to provide bits for identifying additional sub-networks. VLSM is most frequently referred to as subnetting.

Supernetting is different. Supernetting merges several smaller blocks of IP addresses (networks) that are continuous into one larger block of addresses. There can't be any 'holes' in the range. Supernetting is done by borrowing network bits to combine several smaller networks into one larger network.

For instance, a class 'C' block has 256 possible addresses in it. This block could be split into two classless blocks of 128 addresses each by borrowing a single bit from the host portion of the class 'C' address.

The mask and address above combine to give you a range of addresses from 192.64.123.0 through 192.64.123.255.

The mask and address above combine to give you a range of addresses from 192.64.123.0 through 192.64.123.127.

The mask and address above combine to give you a range of addresses from 192.64.123.128 through 192.64.123.255.

Note that the mask used to split the class 'C' network into two smaller subnets is the same, but note that the starting address is different. The first subnet starts with a zero in the last octet, while the second subnet has a starting address of 128 in the last octet.