VLSM

VLSM stands for Variable Length Subnet Mask where a subnet design uses more than one mask for different subnets for single class A, B, C or a network

It is also called subnetting of a subnet

In FLSM (Fixed Length Subnet Mask) all the subnets have equal size but in VLSM, the subnet's size is variable and can also have a variable number of hosts.

This also helps to reduce the wastage of IP addresses.

 To understand VLSM in a better way let us take an example

If our requirement for the network is something like this 👇 

We want to allocate 300.2.1.0. to three different networks

1. We need 120 (50%) addresses for the network A
2. We need 60 (25%) addresses for the network B
3. We need 60 (25%) addresses for the network C

Now if we use FLSM (Fixed Length Subnet Mask), we have to assign 128 bits and 8 bytes would be left but for network B and network C we will have no other option but to assign 128 bits and it is fixed. This would have resulted in the wastage of a large number of IP Addresses.

Now we have to find a way to reduce the wastage of IP addresses and that way is to use VLSM (Variable Length Subnet Mask)

• Address - 300.2.1.0
• Binary - 100101100.00000010.00000001.00000000
• Net ID - 24 bits
• Mask - 255.255.255.0

• For network A - 300.2.1.0
• Binary - 100101100.00000010.00000001.00000000
• Net ID - 25 bits
• Mask - 255.255.255.128

• For network B - 300.2.1.0
• We will allow 6 bits as the number of possible hosts will be 2⁶  which is 64 -2 = 62 hosts 
• Binary - 100101100.00000010.00000001.10000000
• Net ID - 26 bits
• Mask - 255.255.255.192

• For network C - 300.2.1.0
• We will allow 6 bits as the number of possible hosts will be 2⁶  which is 64 -2 = 62 hosts 
• But to avoid ambiguity with the network B address we will make 7th bit from last as 1
• Binary - 100101100.00000010.00000001.11000000
• Net ID - 26 bits
• Mask - 255.255.255.128

Now if you are preparing for some kind of exam then you might want to learn about converting a decimal IP address to binary. But if not then you can use the converter and save yourself from the hassle.

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