VLSM (Variable Length Subnet Masking)
Course Contents
1. Introduction
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.

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.

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.
2. FLSM Approach
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.

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.

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.
3. What is VLSM?
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:
- Start with the office that needs the most IP addresses (in our case, Office A with 50 hosts)
- We create the smallest subnet that can hold those hosts
- Then we move to the next biggest office, and repeat
- We continue assigning subnets from largest to smallest, using only what’s left in the original range

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)

The remaining range 92.1.1.128 92.1.1.255 is now free for future use.
4. FLSM vs VLSM
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:
Aspect | FLSM | VLSM |
---|---|---|
Subnet Size | Fixed for all | Varies per subnet |
IP Efficiency | Low – unused IPs wasted | High – optimized allocation |
Complexity | Easy to implement | Requires careful planning |
Use Case | Uniform environments | Realistic network requirements |
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.