VLSM Planner
Plan variable-length subnets by specifying host requirements. The algorithm assigns optimally-sized, correctly-aligned subnets.
Plan variable-length subnets by specifying host requirements. The algorithm assigns optimally-sized, correctly-aligned subnets.
VLSM (Variable Length Subnet Masking) is a subnetting technique that allows you to use different prefix lengths within the same network. Unlike fixed-length subnetting where every subnet is the same size, VLSM assigns the smallest subnet that fits each requirement, minimizing wasted address space.
For example, if you have a /24 network (256 addresses) and need subnets for 100, 50, and 10 hosts, VLSM would assign a /25 (126 usable), a /26 (62 usable), and a /28 (14 usable), using only 208 of 256 addresses with 81.3% utilization. Fixed halving would waste far more space.
VLSM is an IPv4 address-conservation technique. Its whole purpose is to avoid wasting scarce IPv4 addresses by sizing each subnet to its host count. IPv6 has so much address space that every subnet is conventionally a fixed /64, so there is nothing to conserve. To divide an IPv6 block into smaller prefixes, switch the subnet calculator to IPv6 and use split/join instead.
Learn more about VLSM in our VLSM Subnetting Guide, or review the Subnet Cheat Sheet for all prefix sizes.