IPv4 Variable-Length Subnet Masks

Traditionally, subnets were divided by address class. Subnets had either 8, 16, or 24 significant bits, corresponding to 2²⁴, 2¹⁶, or 2⁸ possible hosts. As a result, an entire /16 subnet had to be allocated for a network that required only 400 addresses, wasting 65,136 (2¹⁶ – 400 = 65,136) addresses.

To help allocate address spaces more efficiently, variable-length subnet masks (VLSMs) were introduced. Using VLSM, network architects can allocate more precisely the number of addresses required for a particular subnet.

For example, suppose a network with the prefix 192.14.17/24 is divided into two smaller subnets, one consisting of 18 devices and the other of 46 devices.

To accommodate 18 devices, the first subnet must have 2⁵ (32) host numbers. Having 5 bits assigned to the host number leaves 27 bits of the 32-bit address for the subnet. The IP address of the first subnet is therefore 192.14.17.128/27, or the following in binary notation:

 11000000 . 00001110 . 00010001 . 100xxxxx

The subnet mask includes 27 significant digits.

To create the second subnet of 46 devices, the network must accommodate 2⁶ (64) host numbers. The IP address of the second subnet is 192.14.17.64/26, or

 11000000 . 00001110 . 00010001 . 01xxxxxx

By assigning address bits within the larger /24 subnet mask, you create two smaller subnets that use the allocated address space more efficiently.

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