Variable Length Subnet Mask (VLSM)

The rapid growth of the almighty Internet help create the fear that very soon we will run out of IP address space. It had developed from mere talk to reality after a careful look at the 32-bit IPv4 addressing structure. After the addresses laid aside for private or research purposes, we were left with a mere classful class C address with 254 host space and class B with 65,534 host address spaces.

In recent years, the number of hosts on the Internet has grown rapidly, from 159,000 in October 1989, to over 72 million by the end of the millennium. According to Cisco, in January 2007, there were over 433 million hosts on the Internet.

Those that make things happen at the IETF (Internet Engineering Task Force) found out that the One fundamental cause of this problem was the lack of flexibility. Based on this, they introduced Classless Inter-Domain Routing (CIDR), which used Variable Length Subnet Masking (VLSM) to help conserve address space. With the introduction of CIDR and VLSM, ISPs (Internet Service Providers) could now assign one part of a classful network to one customer and different part to another customer and network administrators had to use additional subnetting skills. That of course means IP addressing is no longer limited to a /8, /16, or /24 subnet mask. Blocks of IP addresses could be assigned to a network base on the requirements or individual need rather than by class. VLSM is simply subnetting a subnet. Subnets can be further subnetted in multiple levels.

How to work out VLSM

This is the most tricky and important part, especially for a would be network administrator. Lets cut the definition and get practical:

Network A – 26 hosts

Network B – 12 hosts

Network C – 6 hosts

Instead of allocating network:

A –

B –

C –

Network address /24 can be broken down – subnetted according to individual network requirements: will provide network A address space from – (30 hosts) will provide network B address space from – (14 hosts) will provide network C address space from – (8 hosts)

By using VLSM above, we minimize the waste of IP addresses.

Source by Chika Nwokeoma