The IP subnet calculator, in the browser SUBNET
The calculator a network engineer keeps open all day. One bit wrong in a mask puts the hosts on the wrong network, so work it out exactly: networks, ranges, splits and VLSM, for IPv4 and IPv6.
Address and prefix
Split into equal subnets
VLSM plan
One segment per line, as "name 50": how many hosts each needs. They are packed into the block above, largest first.
How it works
Why a calculator
Subnetting is simple arithmetic that is unforgiving: one bit wrong in a mask and traffic goes nowhere. SUBNET does it exactly (32-bit for IPv4, 128-bit for IPv6), including the /31 and /32 special cases and VLSM packing, the part it is easiest to get wrong by hand. It never touches the network; it is pure arithmetic on what you type.
Doing this often? OBSCURA Umbra edits a folder of code and configuration where it is, and runs JavaScript, TypeScript and Python over it in a sandbox. On its own, US$4 a month; with everything else, US$9 a month or US$79 a year. 7 days free.