Variable Length Subnet Masking (VLSM) Explained

The Problem with Fixed Subnets

Imagine you have been given 10.10.0.0/22 — 1,024 addresses — and need to set up four network segments:

Office LAN: 200 devices
Wi-Fi: 80 devices
Server VLAN: 15 devices
Management network: 5 devices

If you split the /22 into four equal /24s (256 addresses each), every segment gets 254 usable addresses. Your management network uses 5 of those 254 — wasting 249. Multiply that across a larger network and the waste becomes significant: unused addresses you can never reclaim for other subnets.

VLSM (Variable Length Subnet Masking) solves this by letting each segment use a prefix length that matches its actual requirements.

Choosing the Right Prefix Length

The rule is: find the smallest subnet that fits your required number of hosts. Since usable hosts = 2^(host bits) − 2, work upwards from your requirement:

Hosts needed	Minimum prefix	Usable hosts
1 – 2	/30	2
3 – 6	/29	6
7 – 14	/28	14
15 – 30	/27	30
31 – 62	/26	62
63 – 126	/25	126
127 – 254	/24	254

For the example above: office LAN needs 200 → /24, Wi-Fi needs 80 → /25, servers need 15 → /27, management needs 5 → /29.

The Golden Rule: Allocate Largest First

Always allocate your largest subnets first. Each subnet's network address must align on a boundary that is a multiple of its own size. If you start with a small subnet and then try to fit a larger one beside it, the larger subnet's address may not align correctly — leaving gaps you cannot use.

Largest-first guarantees every subnet lands on a clean boundary and no space is orphaned.

A Worked Example

Starting with 10.10.0.0/22 (addresses 10.10.0.0 through 10.10.3.255), allocating largest-first:

10.10.0.0/22  ─ 1,024 addresses total
│
├── 10.10.0.0/24    Office LAN      254 usable  (200 needed)
│                   10.10.0.1 – 10.10.0.254
│
├── 10.10.1.0/25    Wi-Fi           126 usable  (80 needed)
│                   10.10.1.1 – 10.10.1.126
│
├── 10.10.1.128/27  Server VLAN      30 usable  (15 needed)
│                   10.10.1.129 – 10.10.1.158
│
├── 10.10.1.160/29  Management        6 usable  (5 needed)
│                   10.10.1.161 – 10.10.1.166
│
└── 10.10.1.168 – 10.10.3.255       Unallocated — available for future growth

Total addresses consumed by the four subnets: 416. Total wasted within those subnets: 111 (the unused addresses in each block). If we had used four equal /24s, the waste would have been 735 — over six times more.

Leave room to grow. Do not allocate right up to the edge of your parent block. Reserve unallocated space for future subnets — a new VLAN for a department, a dedicated security camera network, a VoIP segment. It is far easier to plan for growth upfront than to renumber later.

Point-to-Point Links

Router-to-router links only ever need two addresses — one for each end. Use /30 (4 addresses, 2 usable) rather than wasting a /24 on a link with two interfaces. Modern networks often use /31 for these links, which is supported by most enterprise routers and gives 2 usable addresses with no network or broadcast overhead.

Keeping Track

VLSM planning on a small network is manageable on paper. At 20+ subnets it becomes error-prone: subnets overlap, gaps appear, and the next person to join the team cannot decode the spreadsheet.

An IPAM tool tracks allocations, shows gaps visually, warns about overlaps, and gives you a running view of utilisation in each block — so the network documentation stays accurate without manual effort.