IP Subnet Calculator
Work out the network address, broadcast address, host range and mask of any IPv4 or IPv6 CIDR — and check whether an IP belongs to it. Nothing leaves your browser.
Also available as a desktop app for Windows.
About this tool
Every IP network is a range of addresses, and the shorthand for writing that range down is CIDR: an address, a slash, and a number. 192.168.1.0/24 means "the 256 addresses whose first 24 bits are 192.168.1". That is the whole idea — and it is also why the arithmetic is easy to get wrong, because the boundaries only line up on powers of two.
This page does four things with that arithmetic. It breaks a block down into the numbers you actually need — network address, broadcast address, the first and last usable host, the netmask and its wildcard twin. It tells you what kind of address a block is, so you can see at a glance whether it is private space or something routable on the internet. It checks whether a given IP sits inside a given subnet. And it turns a plain first-to-last range back into the shortest list of CIDR blocks that covers it exactly.
IPv4 and IPv6 are both handled, and both are computed the same way — as plain integers of 32 or 128 bits. Everything runs in your browser: no address you type is sent anywhere, and nothing is logged.
Frequently asked questions
- What does the number after the slash mean?
- It is how many of the leading bits belong to the network — the rest identify hosts inside it. /24 keeps 24 bits for the network and leaves 8 for hosts, which is 2^8 = 256 addresses. /16 leaves 16 host bits (65,536 addresses), /25 leaves 7 (128 addresses), and so on. A bigger number means a smaller network.
- Why is the usable host count two less than the total?
- On an ordinary IPv4 subnet the first address is reserved to name the network itself and the last is the broadcast address that reaches every host at once, so neither can be assigned to a machine. A /24 therefore holds 256 addresses but only 254 hosts. That is also why a /30 (4 addresses) is the smallest block worth using for a normal point-to-point link — it leaves exactly 2.
- Do /31 and /32 behave differently?
- Yes, and this is where many calculators quietly give the wrong answer. A /32 is a single address and a /31 holds two, so the usual "subtract 2" rule would leave 0 usable hosts — clearly nonsense. RFC 3021 fixes this: on a /31 both addresses are usable, which is why modern point-to-point links use them. This tool reports 1 usable address for a /32 and 2 for a /31.
- What is a wildcard mask for?
- It is the netmask flipped upside down: every bit that is 1 in the mask becomes 0, and vice versa. Routers and access lists often want the wildcard form — 192.168.1.0 with a 0.0.0.255 wildcard means "anything ending in 192.168.1". Computers want the netmask; routers want the wildcard. This page shows both so you never have to invert one by hand.
- How is IPv6 different?
- Mostly in scale, and in one detail that trips people up. An IPv6 address is 128 bits, so even a /64 — the standard size for one LAN — holds about 1.8 × 10^19 addresses. There is also no broadcast address in IPv6 at all; the job it did is handled by multicast. So this tool shows a "last address" for IPv6 instead of inventing a broadcast that does not exist.
- Is anything I type sent anywhere?
- No. There is no server involved in the calculation — the whole thing is integer arithmetic running on your own device. That also means the page keeps working with no network connection, and that a private range you paste in never leaves your machine.