VLSM Planner

Plan IPv4 subnets from host requirements using Variable Length Subnet Masking.

What is VLSM?

Variable Length Subnet Masking (VLSM) lets you use subnets of different sizes inside one parent network. Instead of splitting 10.0.0.0/24 into four equal /26 blocks, VLSM gives a 100-host department a /25 and a 20-host server room a /27 — each subnet only as large as it needs to be.

Why use VLSM?

Equal-size subnetting wastes addresses when departments have very different sizes. A 300-user floor needs 512 addresses, but a 10-address camera VLAN forced into the same block wastes hundreds. VLSM matches block size to the actual requirement, so more of the parent range stays free for future growth.

How this planner allocates subnets

Each requirement gets the smallest valid subnet that fits its host count (network and broadcast addresses are reserved).
Subnets are allocated largest first, so big blocks get the cleanest, best-aligned space.
Every subnet starts on a correct network boundary — a /26 block can only start at an address divisible by 64.

Example

In 10.0.0.0/24, 100 hosts needs a /25 (128 addresses, 126 usable) and 50 hosts needs a /26 (64 addresses, 62 usable). The planner places them at 10.0.0.0/25 and 10.0.0.128/26.

Important limitation

This tool uses traditional IPv4 subnet semantics: the network and broadcast addresses are reserved, so 1–2 hosts both get a /30 (2 usable hosts). It does not use RFC 3021 /31 point-to-point subnets or /32 host routes in planning.