VLSM (Variable Length Subnet Masking)

What is VLSM in networking, and why does it matter?
To truly understand how Variable Length Subnet Masking (VLSM) works in networking, let’s walk through a real-world scenario.

Figure 1 – Public IP Block 92.1.1.0/24

Imagine you're working as a network engineer, and your company just acquired the public IP block 92.1.1.0/24.

This subnet provides a total of 256 IP addresses from 92.1.1.0 to 92.1.1.255
Your task is to design the subnetting plan for four new office branches below.

Figure 2 – Network Topology for Branch Offices

Each office needs its own subnet.

Your first instinct might be to simply split the block into four equal subnets, one per office. That seems fair…
but is it really the most efficient way?

Let’s explore this first option.

A common first approach to subnetting is to divide a network block into equal-sized subnets, regardless of the number of hosts each subnet actually needs.

This is known as Fixed Length Subnet Masking (FLSM).

In our example, we start with the 92.1.1.0/24 network, which contains 256 IP addresses.

Figure 3 – Fixed Length Subnet Masking (FLSM) Example

We split this block into four /26 subnets, assigning one to each office:

• Office A: 92.1.1.0/26

• Office B: 92.1.1.64/26

• Office C: 92.1.1.128/26

• Office D: 92.1.1.192/26

Each /26 provides 64 IP addresses, which is far more than Offices C and D actually need.

Figure 4 – FLSM Subnet Allocation for Offices

The Problem with FLSM

FLSM is simple, but inefficient:

• Office C and Office D only need 10 IPs, but each still receives 64.

• That’s a clear waste of address space, especially in IPv4 where every IP counts.

We need a smarter strategy, one that gives each office only what it actually needs.