OK this doc is good to understand. But how to find the TOC of this cisco document?? If I go one level up, it says "No page Found"
Briefly said, VLSM as the name suggests allows you to use multiple (variable) subnet masks in different parts of your network. Suppose you have 3 networks one with 100 hosts, the second with 10 host and the 3rd is with only 5. The 100 hosts network will need at a least 7 bits in the hosts bits, Thats a subnet mask of /25 (or less: /24, /23, /22,,,,,) to accommodate for all hosts. However, The 10 hosts network will need only 4 bits (/28 ) and the 5 hosts will need 3 bits (/29).
To use a single subnet mask for all the nets, it has to be enough for the largest (100 host). So /25 will have to be used for all. BUT thats a big waste in the address space, isn't it?
VLSM allows you to use /25 for the 100 hosts. /28 for the 10 hosts. and /29 for the 5.
However, Not all routing protocols support VLSM. RIP and IGRP don't, RIPv2, EIGRP and OSPF do support VLSM. The reason why RIP and IGRP don't is because they DO NOT send the subnet mask with the routing updates, they just assume it's class-full (/8, /16 or /24).
This is very brief. I hope I did not confuse you. Check here for further explanation, there is a VLSM example:
www.cisco.com/en/US/tech/tk365/technolog...f5.shtml#vlsmexample
