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.

A first idea is to split the block into four equal /26 subnets, one per office.
This is called Fixed Length Subnet Masking (FLSM): the same size for everyone, so Offices C and D would receive 62 usable hosts for 10 devices each.

Is there a way to give each office only what it needs?

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

Instead of assigning the same subnet size to every office, we allocate only the number of IP addresses each location actually requires.

How It Works

With VLSM, we do not 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)

• Create the smallest subnet that can support those hosts

• Move to the next largest requirement

• Continue assigning subnets from largest to smallest, using only the remaining available address space

Figure 3 – Variable Length Subnet Masking (VLSM) Allocation

After applying VLSM to the 92.1.1.0/24 network, we obtain:

• Office A: 92.1.1.0/26 (64 IP addresses)

• Office B: 92.1.1.64/27 (32 IP addresses)

• Office C: 92.1.1.96/28 (16 IP addresses)

• Office D: 92.1.1.112/28 (16 IP addresses)

Figure 4 – VLSM Subnet Allocation for Offices

The remaining range 92.1.1.128 – 92.1.1.255 remains unused and available for future expansion.
This demonstrates the efficiency of VLSM: each office receives only what it needs, and no address space is wasted unnecessarily.

Challenge

Complete the interactive scenario below by allocating the optimal subnet sizes for all four branch offices. Make sure to satisfy each office's host requirements without wasting address space. Once solved, copy the generated flag.

Enter the flag you received upon successfully completing the VLSM allocation scenario to validate this challenge.

Implementing the VLSM Design in Cisco IOS

Now let’s apply this VLSM plan to a router configuration.

Office A – 92.1.1.0/26

Router# conf t
Enter configuration commands, one per line.  End with CNTL/Z.
Router(config)# interface g0/0
Router(config-if)# ip address 92.1.1.1 255.255.255.192
Router(config-if)# no shutdown
Router(config-if)# exit

Office B – 92.1.1.64/27

Router(config)# interface g0/1
Router(config-if)# ip address 92.1.1.65 255.255.255.224
Router(config-if)# no shutdown
Router(config-if)# exit

Office C – 92.1.1.96/28

Router(config)# interface g0/2
Router(config-if)# ip address 92.1.1.97 255.255.255.240
Router(config-if)# no shutdown
Router(config-if)# exit

Office D – 92.1.1.112/28

Router(config)# interface g0/3
Router(config-if)# ip address 92.1.1.113 255.255.255.240
Router(config-if)# no shutdown
Router(config-if)# end

To verify the configuration:

Router# show ip route
C    92.1.1.0/26 is directly connected
C    92.1.1.64/27 is directly connected
C    92.1.1.96/28 is directly connected
C    92.1.1.112/28 is directly connected

This confirms that multiple subnet masks can coexist within the same original /24 block, which is exactly what VLSM allows.

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

The block you split in this lesson was public, bought by the company.
The next lesson shows the ranges any company can use inside its network without buying anything: the private IPv4 addresses.