Free tool
VLSM Subnet Divider
Split a network into smaller subnets of the sizes you actually need, and get the ranges, masks and usable hosts for each one without overlaps.
| Need | Subnet | Mask | Usable range | Broadcast | Hosts |
|---|
Why variable length masks exist
If every subnet is the same size you waste most of your address space. A point to point link between two routers needs two addresses and gets 254 if you hand it a /24. VLSM lets each subnet be only as large as it needs to be, which is how a single /24 can serve a whole small office with room left over.
The allocation has to go largest first. Subnets must start on a boundary that matches their own size, so placing a small one first leaves gaps that a larger one can no longer fit into. The tool sorts your list automatically and allocates downwards so nothing overlaps.
Remember the two addresses lost per subnet to the network and broadcast. If you need fifty hosts you need a /26, which gives sixty two usable, because a /27 only gives thirty. Leave room for growth as well, since renumbering a live network afterwards is a far longer job than getting the plan right on paper.