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What is a Variable Length Subnet Mask?

Variable Length Subnet Masks (VLSM), is a non-classful mask applied to a classful IP default mask. The defaults are:
Address Range Mask

Class A 1.0.0.0.0 - 126.255.255.255 255.0.0.0 (8 bit mask)
Class B 128.0.0.0 - 191.255.255.255 255.255.0.0 (16 bit mask)
Class C 192.0.0.0 - 223.255.255.255 255.255.255 (24 bit mask)

All masks of other values for each of the classes are of variable length.

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