Subnet & CIDR
Subnet without the guesswork
Ranges, a prefix for a device count, and an overlap check.
Nothing is uploaded. The maths runs in this tab.
Network
IPv4 or IPv6. Any address inside the block works — it is normalised to the network address.
Result
How many devices?
Count everything that takes an address: laptops, phones, printers, APs, cameras, and headroom.
Recommendation
Enter a device count.
Do these overlap?
Both blocks must be the same IP version.
Verdict
Enter two blocks.
- IPv4 & IPv6
- RFC 3021 aware
- Exact big-integer maths
- Runs offline
Why the usable-host counts look unusual
Usable-host counts follow the standards rather than a simple formula: a /31 carries 2 usable addresses for point-to-point links (RFC 3021) and a /32 carries 1, instead of the "minus two" that applies to ordinary subnets.
IPv6 does not reserve a broadcast address, so every address in the prefix is usable.
Questions people actually ask
Subnet maths is easy to get subtly wrong, and the subtle errors are the ones that reach production. This does the arithmetic exactly, in big integers, and follows the standards rather than the usual shortcut formula.
What prefix do I need for 500 devices?
Enter the device count and the recommender returns the smallest prefix that fits, with the headroom it leaves. It accounts for the addresses you cannot assign, so the number it gives is usable hosts and not raw address count.
Why does a /31 show two usable addresses?
Because RFC 3021 says so. A /31 carries two usable addresses for point-to-point links and a /32 carries one, instead of the 'minus two' that applies to ordinary subnets. Many calculators get this wrong and report zero.
Does it support IPv6?
Yes. IPv6 arithmetic uses BigInt so the counts are exact rather than rounded through floating point. IPv6 reserves no broadcast address, so every address in the prefix is usable.
Can it tell me if two subnets overlap?
Yes — the overlap check takes two blocks and reports whether one contains the other, where the shared range starts and ends, and how many addresses are involved. Worth running before committing an address plan.
Is anything sent to a server?
No. It is pure arithmetic in the browser, so your internal address plan stays internal.
Part of nine free KAIORB instruments — all in-browser, none of them upload your files.