What is VLSM?
VLSM (Variable Length Subnet Mask) is simply a subnetting a subnet. After the network address is subnetted, those subnet can be further subnetted.VLSM is a sub-subnetting.
In routing protocols RIP v1 or IGRP not use VLSM means there is no field defines for subnet information in these protocols. This is called classfull routing. RIP and IGRP both are considered as classful routing protocol.
Classless routing protocol support the VLSM means these protocols define the field for the subnet information. Therefore we can use the VLSM with the classless routing protocols like RIPv2,EIGRP or OSPF.With the VLSM we can have different subnet mask for the different subnets.Classfull routing protocol require only single network and use the same subnet mask, example- Class C ip address 192.168.10.0 would be only one subnet mask that is 255.255.255.0.VLSM allow a single autonomous system to have network with different subnet mask VLSM allows an organization to use more than one subnet mask within the same network address space. VLSM is often referred to as “subnetting a subnet”, and can be used to maximize addressing efficiency.
For the example we can take class A network address- 10.0.0.0/8 which has subnet mask 255.0.0.0 and subnetting the network it gives 256 subnet using subnet mask /16-
10.0.0.0/16
10.1.0.0/16
10.2.0.0/16
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10.255.0.0/16
These are the subnets of the network 10.0.0.0/8 and these are having the subnet mask /16.If we take the one subnet for example 10.1.0.0/16 and subnetted it again with a /24 mask –
10.1.1.0/24
10.1.2.0/24
10.1.3.0/24
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10.1.255.0/24
These are the subnet of network 10.1.0.0/16 and use the subnet mask /24.This is subnetting a subnet and called VLSM.
The use of VLSM and CIDR not only used to prevent the address waste but also define route aggregation and route summarization.Route summarization is only possible if the router in the network use classless routing protocol like EIGRP,OSPF.Classles routing protocol carry a prefix that consist IP address and the subnet mask. VLSM allows for the summarization of routes and increases flexibly by basing the summarization entirely on the higher-order bits shared on the left, even if the networks are not contiguous.
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