Variable Length Subnet Masking (VLSM) allows you to create subnets of different sizes within a single network, optimizing IP address utilization and reducing waste by up to 80%.
Automatically calculates the smallest subnet mask that fits your requirements
Minimize IP address waste by creating right-sized subnets
Intelligent algorithm allocates largest subnets first
Variable Length Subnet Masking (VLSM) allows you to create subnets of different sizes within a single network, optimizing IP address utilization and reducing waste.
All subnets are the same size, leading to significant IP address waste when subnet requirements vary.
Example: Using /26 for both 50 hosts and 5 hosts wastes 57 addresses
Each subnet gets exactly the right size, maximizing efficiency and minimizing waste.
Example: /26 for 50 hosts, /29 for 5 hosts - optimal utilization
Our VLSM calculator uses the industry-standard approach: sort subnet requirements by size (largest first), then allocate each subnet using the smallest possible subnet mask. This ensures optimal address space utilization and prevents fragmentation issues.
Variable Length Subnet Masking lets you carve one address block into subnets of different sizes, instead of splitting it into equal pieces. A /24 divided the fixed way gives you four /26s whether or not each segment needs 62 hosts. With VLSM you can hand a 100-host LAN a /25, a 50-host LAN a /26, and each router-to-router link a /30, drawing every subnet from the same /24 and leaving the remainder free for later.
Always largest first. Allocate the subnet with the biggest host requirement at the start of the block, then the next biggest, and so on down to your /30 or /31 point-to-point links. Working largest-to-smallest keeps every subnet aligned on its natural boundary and prevents the fragmentation you get when a small subnet is placed where a large one would have fit. This calculator applies that ordering automatically once you enter your host counts.
Because two addresses in every IPv4 subnet are unusable: the network address and the broadcast address. A /27 provides 32 total addresses and therefore 30 usable ones, which is short of 50. A /26 provides 64 total and 62 usable, so it is the smallest prefix that fits. The general rule is to find the smallest power of two that is at least your host count plus two.
Yes, on equipment that supports RFC 3021. A /31 gives two addresses and treats both as usable host addresses, which is exactly what a router-to-router link needs, halving the waste compared with a /30. Support is widespread on modern routers but not universal, so confirm your platform handles it before standardising on /31s; otherwise a /30 with two usable addresses remains the safe choice.
Classless protocols do, because they carry the subnet mask in their routing updates. OSPF, EIGRP, IS-IS, RIPv2 and BGP all qualify. The original RIPv1 and IGRP are classful and do not advertise masks, so they cannot represent subnets of differing lengths within one classful network. Any modern design will be running a classless protocol, so VLSM is generally safe to assume.