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Subnet calculator

Network, mask, broadcast, host range and the bits behind them.

It runs entirely in this browser tab: the addresses, names and passwords you type are never uploaded, and the tool makes no network request while it works. Open your browser's network panel and try it — nothing leaves the page.

Settings

Show the bits: The address and the mask written out in binary, which is where a prefix stops being a mystery.
Split into: A prefix length to cut this network into. Nought leaves it whole. A number from 0 to 128.

How to use it

Paste your text into the box above, or open a file.
Adjust the settings beside it until the result is what you wanted.
Copy the result, or save it as a file. The result updates as you type.

Questions

Does this look anything up?: No, and that is the point of the group. Nothing here resolves a name, queries a registry or opens a connection. Three tools would want a table bigger than the whole application — the vendor prefixes, the routing table, the full port register — and each of them takes the table from you and says so rather than answering "unknown" about something that exists.
Why does the subnet calculator show the bits?: Because a prefix stops being a mystery the moment you see it written out. It is also where the two lengths everybody gets wrong become obvious: a /31 is a point-to-point link with two usable addresses and no broadcast, and a /32 is one host rather than minus one.
Can I paste something confidential in?: Yes, and that is why the group is built this way. An internal addressing plan, a DKIM private key's public half, a Wi-Fi password and an authorisation header are all things you would think twice about typing into a website. None of them is sent anywhere, and the request builder hides credentials by default so that what you paste into a ticket is safe to.
Is what I type uploaded anywhere?: No. This page carries the whole tool as WebAssembly and runs it on your own device. You can watch your browser's network panel while you use one.

Other tools for blocks of addresses

IPv6 subnet calculator
The range, both notations and how many networks fit inside.

CIDR range calculator
A range as the fewest blocks, blocks joined, and the overlap of two sets.

VLSM planner
One network cut into the subnets you asked for, largest first.

Address in subnet check
Which of your blocks each address falls in, and which fall in none.

IPv4 in IPv6
The four ways a v4 address is written inside a v6 one, both directions.

All thirty tools

Subnet calculator

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