What is VLSM

Quesion: What is  Variable Length Subnet Mask (VLSM) ?

Ans: VLSM (Variable Length Subnet Mask) is used to reduce the wastage of IP address space, to reduce the congestion. In VLSM, different length of subnet mask is used and more network can be addressed.

e.g.  255.255.255.128
        255.255.255.192
        255.255.255.224  ...........
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There are wastage of IP addresses in classful IP address.

Now take one  example

1) There is a class C network in which 100 host needs to be connected so what will be the best subnet mask?

First consider Classful addressing, Default subnet mask for class C is 255.255.255.0 in which 254 hosts can be connected. Here we can find that there is wastage of 254-100=154 IP Address.

Now consider VLSM addressing for this network.
We calculate the off bits for calculating subnet mask.

Number of Host = 2^off bits -2
100=2^off bits-2
100+2=2^off bits
102=2^off bits
off bits = = 7  ( 2^off bits=128 which is nearest to 102)

Here we calculate the nearest off bits as 7 in which 126 host can be connected .
So the wastage of IP address is 126-100=26. which is very less as compared to 154.

So the  subnet mask will have 7 bits off.
subnet mask = 11111111.11111111.11111111.10000000
                         255.255.255.128
So for 100 hosts this 255.255.255.128 VLSM is used instead of 255.255.255.0