VLSM stands for Variable Length Subnet Masking. It’s a method used in computer networking to divide an IP address space into subnets of different sizes, depending on how many devices (hosts) each subnet needs.

Why do we need VLSM?

Imagine we are managing a school’s network. We have:

100 computers in the computer lab 50 in the admin block 10 in the library 2 for security cameras

If we divide the IP address space into equal parts (called Fixed Subnetting), each group might get more IP addresses than it actually needs. That’s wasteful.

With VLSM, we can give:

128 addresses to the lab (more than 100) 64 to the admin block 16 to the library 4 to the cameras

So, it save IP addresses by assigning only what is needed.

How does VLSM work?

• Start with a large IP block (like 192.168.1.0/24 which has 256 addresses).

• List networks based on how many IPs each one needs.

• Allocate subnets from largest to smallest, using subnet masks that fit the number of devices.

• Adjust the subnet mask (that’s the “variable” part) to create subnets of different sizes.

Example:

Let us split 192.168.1.0/24 into 3 subnets:

50 devices 20 devices 10 devices

we could divide like this:

First subnet: 192.168.1.0/26 → 64 addresses (for 50 devices) Second: 192.168.1.64/27 → 32 addresses (for 20 devices) Third: 192.168.1.96/28 → 16 addresses (for 10 devices)

Benefits of VLSM:

• Efficient use of IP addresses

• More flexible network design

• Scalable for future changes

VLSM allows network administrators to divide an IP address block into subnets of different sizes, based on how many IP addresses each group actually needs. This is more efficient than fixed-size subnetting, which can waste IPs by giving every group the same number of addresses.

With VLSM, subnet masks are adjusted to fit the size of each network, helping to conserve IP space and optimize network design. For example, a school network could assign a larger subnet to a computer lab and smaller ones to offices or security systems—all within the same IP range.