Step 9 — VLSM (Variable Length Subnet Masking)
VLSM allows you to use different subnet masks within the same network, optimizing IP address usage. Instead of wasting addresses with fixed-size subnets, you right-size each subnet for its needs.
Why VLSM Matters
Without VLSM (wasteful):
You have a /24 network and need subnets for: 100 hosts, 50 hosts, 20 hosts, and 2 hosts.
Using fixed /26 subnets (62 hosts each): You can't even fit the 100-host subnet! Using fixed /25 subnets (126 hosts each): Only 2 subnets possible, massive waste.
With VLSM (efficient):
- 100 hosts → /25 (126 hosts)
- 50 hosts → /26 (62 hosts)
- 20 hosts → /27 (30 hosts)
- 2 hosts → /30 (2 hosts)
All fit within a single /24!
The VLSM Process
- Sort requirements from largest to smallest
- Allocate the largest subnet first from the start of your address space
- Allocate the next largest from where the previous ended
- Repeat until all subnets are allocated
- Verify no overlaps and addresses remain within your block
Worked Example
Given: 192.168.1.0/24
Create subnets for:
- LAN A: 100 hosts
- LAN B: 50 hosts
- LAN C: 25 hosts
- WAN link: 2 hosts
Step 1: Sort by size (largest first)
- LAN A: 100 hosts
- LAN B: 50 hosts
- LAN C: 25 hosts
- WAN link: 2 hosts
Step 2: Determine subnet sizes needed
- 100 hosts → need /25 (126 hosts, next power of 2 above 100+2)
- 50 hosts → need /26 (62 hosts)
- 25 hosts → need /27 (30 hosts)
- 2 hosts → need /30 (2 hosts)
Step 3: Allocate addresses
| Subnet | CIDR | Network | Host Range | Broadcast |
|---|---|---|---|---|
| LAN A | /25 | 192.168.1.0 | .1 - .126 | .127 |
| LAN B | /26 | 192.168.1.128 | .129 - .190 | .191 |
| LAN C | /27 | 192.168.1.192 | .193 - .222 | .223 |
| WAN | /30 | 192.168.1.224 | .225 - .226 | .227 |
Remaining addresses: 192.168.1.228 - 192.168.1.255 (available for future use)
Visual: Address Space Usage
192.168.1.0/24 Address Space:
Practice Problem
Given: 10.10.0.0/16
Create an addressing scheme for:
- Headquarters: 4,000 hosts
- Branch Office 1: 500 hosts
- Branch Office 2: 250 hosts
- Data Center: 100 hosts
- Management VLAN: 20 hosts
- 3 WAN links: 2 hosts each
Your design (saved)
Write your chosen CIDRs and the network ranges you’d assign to each segment.
Saved locally in your browser for this device.
Show Solution
Determine subnet sizes:
- 4,000 hosts → /20 (4,094 hosts)
- 500 hosts → /23 (510 hosts)
- 250 hosts → /24 (254 hosts)
- 100 hosts → /25 (126 hosts)
- 20 hosts → /27 (30 hosts)
- 2 hosts × 3 → /30 each (2 hosts)
| Subnet | CIDR | Network | Broadcast |
|---|---|---|---|
| HQ | /20 | 10.10.0.0 | 10.10.15.255 |
| Branch 1 | /23 | 10.10.16.0 | 10.10.17.255 |
| Branch 2 | /24 | 10.10.18.0 | 10.10.18.255 |
| Data Center | /25 | 10.10.19.0 | 10.10.19.127 |
| Management | /27 | 10.10.19.128 | 10.10.19.159 |
| WAN 1 | /30 | 10.10.19.160 | 10.10.19.163 |
| WAN 2 | /30 | 10.10.19.164 | 10.10.19.167 |
| WAN 3 | /30 | 10.10.19.168 | 10.10.19.171 |
Remaining: 10.10.19.172 - 10.10.255.255 for future growth
Common VLSM Mistakes
❌ Not sorting by size first
Always allocate largest subnets first to avoid fragmentation
❌ Overlapping subnets
Each new subnet must start AFTER the previous one ends
❌ Forgetting subnet boundaries
Subnets must start on proper boundaries (multiples of their block size)
Checkpoint
Before moving on, make sure you can:
- Explain why VLSM is more efficient than fixed-length subnetting
- Calculate the correct subnet size for a given host requirement
- Allocate multiple variable-sized subnets without overlaps
- Verify that all subnets fit within your address space