VLSM subnet planner

Give it a base network and how many hosts each subnet needs (comma-separated, with optional names like servers:50). It allocates right-sized subnets, largest first, the way you would on paper but without the arithmetic slips. Runs entirely in your browser.

How the sizes fall out

Each subnet gets the smallest block holding the hosts plus network and broadcast: 50 hosts needs 52 addresses, so a /26 (64). Allocating largest-first keeps every block aligned to its natural boundary with no gaps. Growth is the judgement call the calculator will not make for you: a subnet at 60 of 62 addresses is already a project, so size for the estate you expect, not the one you have.

For a single subnet's details, the plain subnet calculator is quicker, and the maths lives in the subnet cheat sheet.