About
A subnet calculator that shows its work.
IP CIDR Calculator is a free tool for reading and dividing address space — CIDR blocks, subnet masks, host counts and split trees, for IPv4 and IPv6 alike. It is deliberately small, deliberately static, and deliberately offline once the page has loaded.
Client-side only
Every calculation runs in JavaScript in your browser. Addresses you type are never sent anywhere, because there is nowhere to send them — the site is a folder of static files.
One code path
IPv4 and IPv6 share the same BigInt arithmetic. A /24 and a /64 are the same operation on a different address width, so a bug cannot hide in one family and not the other.
Numbers, then prose
Every host count, mask and boundary printed in the copy is produced by the same functions the calculator uses, and checked by the test suite. Nothing is worked out by hand.
No account, no tracking
No sign-up, no analytics, no cookies, no advertising. The one thing stored on your device is whether you prefer the light theme.
Why this exists
Most subnetting tools answer the question you asked and stop there. You type a block, you get a mask, and you are left to trust it. That is fine until the number matters — until a firewall rule, a VPC plan or a route summary depends on whether a /22 really does end at .7.255.
This site is built on the opposite instinct: show the boundary, show the bit that moved, and let the block be divided and re-divided until the plan is visible. The subnet splitter is the centre of it — a CIDR block opened into a tree, where any node can be split again and any value copied with one click.
How the numbers are produced
All address arithmetic lives in a single module. Addresses are parsed into arbitrary-precision integers, so a 32-bit IPv4 address and a 128-bit IPv6 address are handled by the same functions with a different width — no separate IPv6 path that can drift out of step.
That module has a test suite, and the figures quoted in the guides and reference tables are generated from it at build time rather than typed in. If a host count on the subnet cheat sheet were wrong, the same bug would have to be wrong in the calculator too — and the tests would have to miss it.
What it does not do
There is no lookup of any kind. The site cannot tell you who owns an address, where it is located, whether it is reachable, or what is running on it — there is no server to ask. It does arithmetic on the address you type, and nothing else.
It is also not a source of legal or compliance advice about address allocation. For authoritative rules, the RFCs linked in the footer are the actual specification; the CIDR guide is an explanation of them, not a substitute.
Who it is for
Network engineers planning an allocation, cloud engineers sizing a VPC, developers writing an allow-list, students working through subnetting for the first time, and anyone who has been handed a range like 100.64.0.0/10 and needs to know what it covers. No part of it is gated, rate-limited or reserved for a paid tier.
Corrections
If a figure looks wrong, it is worth reporting — a reproducible case is more useful than any feature request. The contact page has the address and what to include.