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.

Variable Length Subnet Masking (VLSM) is a smarter way to divide a network.

Instead of giving the same size to every subnet, we allocate just the amount of IPs each office really needs.

How It Works (Without the Math)

With VLSM, we don't split the address space into equal blocks. Instead, we:

1. Start with the office that needs the most IP addresses (in our case, Office A with 50 hosts)

2. We create the smallest subnet that can hold those hosts

3. Then we move to the next biggest office, and repeat

4. We continue assigning subnets from largest to smallest, using only what's left in the original range

Figure 5 – Variable Length Subnet Masking (VLSM) Allocation

As you can see, each office now receives just what it needs:

• Office A: 92.1.1.0/26 (64 IPs)

• Office B: 92.1.1.64/27 (32 IPs)

• Office C: 92.1.1.96/28 (16 IPs)

• Office D: 92.1.1.112/28 (16 IPs)

Figure 6 – VLSM Subnet Allocation for Offices

The remaining range 92.1.1.128 92.1.1.255 is now free for future use.

In today’s world, IPv4 space is limited and valuable.
Knowing how to subnet efficiently is a crucial skill.

Here’s a comparison of both methods:

Table 1 – Comparison of FLSM and VLSM

Want to try it yourself?

You can use this VLSM Calculator to practice subnetting the 92.1.1.0/24 network into four subnets with 64, 32, 32, and 16 hosts respectively.

Now that you know what VLSM is in networking, it’s time for you to learn how to calculate it yourself.

In the next lesson, you’ll walk through each step to determine subnet sizes, assign ranges, and build an efficient VLSM plan from scratch.