The VLSM Subnet Planner automates Variable Length Subnet Masking, the technique of dividing an IP address space into subnets of different sizes based on the number of hosts each network segment needs. Rather than assigning the same size subnet to every VLAN, VLSM allocates the smallest possible subnet that fits the required hosts, minimising wasted IP address space.

Enter your base network address and CIDR prefix, then add your VLANs with their names and required host counts. The tool sorts VLANs from largest to smallest (the correct VLSM order), assigns the optimal subnet to each one, and shows you the network address, subnet mask, usable host range and broadcast for every VLAN in a single table. The utilisation bar shows what percentage of your address space has been allocated.

The Cisco IOS Interface Config output generates ready-to-paste configuration for each VLAN interface including the correct IP address (first usable host as the gateway), subnet mask and no shutdown command. This saves significant time when configuring a Layer 3 switch or router for a new network design, and reduces the risk of typos in subnet masks during a production change.