IPv4 VLSM - Internet Protocol version 4 (IPv4)

What is IPv4 VLSM?

Sometimes, Internet Service Providers might have to allocate IP subnets of different sizes based on the requirement of customer. One customer can ask Class C subnet of 3 IP addresses and another can ask for 10 IPs. For an ISP, it is not possible to divide the IP addresses into fixed size subnets, but we can subnet the subnets in such a way that it results in minimum wastage of IP addresses.

For example, an administrator have network. The suffix /24 (pronounced as "slash 24") will tell the number of bits used for network address. In this example, the administrator will have three different departments with different number of hosts. Sales department will have 100 computers, Purchase department will have 50 computers, Accounts will have 25 computers and Management will have 5 computers. In CIDR, subnets are of fixed size. By using the same methodology, administrator will not be able to fulfill all the requirements of the network.

Below procedure will show how VLSM can be used for allocating department-wise IP addresses as described in the example.

Step - 1

Make a list of Subnets possible.


Step - 2

Sort the requirements of IPs in descending order (Highest to Lowest).

  • Sales 100
  • Purchase 50
  • Accounts 25
  • Management 5

Step - 3

Allot the highest range of IPs to the highest requirement, assign /25 ( to the Sales department. This IP subnet with Network number will have 126 valid Host IP addresses which satisfy the requirement of the Sales department. Subnet mask used for this subnet will have 10000000 as the last octet.

Step - 4

Allocate the next highest range, so let's assign /26 ( to the Purchase department. This IP subnet with Network number has 62 valid Host IP Addresses which can be easily assigned to all the PCs of the Purchase department. Subnet mask used will have 11000000 in the last octet.

Step - 5

Allocate the next highest range, i.e. Accounts. The requirement of 25 IPs can be fulfilled with /27 ( IP subnet which consist of 30 valid hosts IPs. Network number of Accounts department will be and the last octet of subnet mask is 11100000.

Step - 6

Allocate the next highest range to Management and the Management department will consist of only 5 computers. Subnet /29 with the Mask will have exactly 6 valid host IP addresses. Therefore this can be assigned to Management and the last octet of the subnet mask will contain 11111000.

By using VLSM, the administrators can subnet the IP subnet in such a way that the least number of IP addresses are wasted. Even after assigning IPs to every department, administrator, in this example, will still be left with many IP addresses which would not be possible with CIDR.

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Internet Protocol version 4 (IPv4) Topics