VLSM Subnet Planner

Plan an optimized IP addressing scheme using Variable Length Subnet Masking. Enter your parent block and the number of hosts per segment — the planner assigns the smallest possible subnet to each requirement, minimizing waste.

Parent network block

Subnet requirements

Subnet nameHosts needed

Fill in the requirements and click Plan VLSM

What is VLSM?

Variable Length Subnet Masking (VLSM) allows a network to use different subnet masks for different subnets within the same network. Unlike classful subnetting where all subnets share the same mask, VLSM assigns each subnet exactly the block size it needs.

VLSM design rules

The canonical VLSM algorithm: sort requirements largest-to-smallest, then assign the smallest block that satisfies each requirement in order. This guarantees non-overlapping, contiguous allocation with minimal wasted space.

WAN links and point-to-point subnets

Serial links between routers typically require only 2 IP addresses — one per end. Use /30 (2 usable hosts) for traditional point-to-point links, or /31 per RFC 3021 on modern IOS versions that support it.