Similar to the previous question, this is another favorite fundamental question. VLSM is a feature of OSPF, RIPv2, and BGP that enables classless routing. With classful routing protocols such as RIPv1 or IGRP, every autonomous system uses the same subnet mask. For example, 192.168.16.0, 192.168.17.0, and 192.168.18.0 are all Class C networks and therefore have a /24 or 255.255.255.0 subnet mask.
VLSM allows an autonomous system to support different subnet masks such as 192.168.18.0/26 and 192.168.18.128/25 to support subnets. VLSM also supports “supernetting,” such as describing 192.168.20.0/23 to include all hosts in 192.168.20.0 and 192.168.21.0. Do not be surprised if you are asked to perform a
few subnet and supernet examples.
For more information on VLSM, Google “VLSM site:cisco.com.”