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variable length subnet mask
Last modified: Tuesday, May 16, 2006
Variable Length Subnet Mask, abbreviated as VLSM, is a means to specify a different subnet mask for the same network number on different subnets. With VLSM, a network administrator can use a long mask on networks with few hosts and a short mask on
subnets with many hosts. To use VLSM, the routingprotocol must support it.
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Why Don't RIPv1 and IGRP Support VLSM? (PDF) RIPv1 and IGRP are classful protocols and are incapable of carrying subnet mask information in their updates. Before RIPv1 or IGRP sends out an update, it performs a check against the subnet mask of the network that is about to be advertised and, in case of VLSM, the subnet gets dropped.
