VLSM Calculator

Variable Length Subnet Masking (VLSM) gives each network only the addresses it needs: a /25 for a 100-host LAN, a /30 for a router link, instead of one fixed size for everything. Enter the major network and the hosts each subnet needs, and this calculator allocates them in the standard largest-first order on aligned boundaries, then lists the network, mask, host range, broadcast address and spare addresses for each, plus how much of the block is left.

How to use this tool

1. Enter the address block you have, such as 192.168.1.0/24.
2. List each subnet as "name: hosts", one per line.
3. Keep "largest first" for a gap-free plan (choose "as listed" to keep your order).
4. Copy the table into your network documentation or router configuration.

Formula

block size = 2^⌈log₂(hosts + 2)⌉ · prefix = 32 − log₂(block size) · next network = previous network + previous block size

Two addresses per subnet go to the network and broadcast addresses, hence hosts + 2. Allocating the largest blocks first keeps every block aligned to its own size with no gaps; with RFC 3021 enabled, 2-host links get a /31.

Example

For 192.168.1.0/24 with A = 100, B = 50 and C = 20 hosts: A needs 102 addresses → a block of 128 (192.168.1.0/25); B needs 52 → 64 (192.168.1.128/26); C needs 22 → 32 (192.168.1.192/27). That uses 224 addresses and leaves 32 free from 192.168.1.224. Asking for 300 hosts in a /24 fails: a /24 holds only 254.