Subnet Calculator

Subnet Calculator takes an IPv4 address and a CIDR prefix and works out everything that follows from them: the network address, the broadcast address, the usable host range, the subnet mask and its wildcard. You can type the prefix in its own box or append it to the address as 192.168.1.130/26, which is how it usually appears in configuration.

The prefix is the number of leading bits that identify the network, so a /26 fixes 26 bits and leaves 6 for hosts. That gives 64 addresses, of which 62 are usable: the first is the network address and the last is the broadcast, and neither can be assigned to a machine. Every figure here comes from that single piece of arithmetic, which is why getting the prefix right matters more than anything else in a subnet plan.

Two prefixes break the usual rule and are handled specially. A /31 has no room for a network and broadcast pair, so RFC 3021 defines both of its addresses as usable for point-to-point links. A /32 is one address, a single host route. A formula that simply subtracts two reports zero and minus one for these, which is wrong and causes real confusion when reading a routing table.

The page shows its working, not just its answer. Where there is a formula it is printed with your own numbers substituted in, so you can check the result rather than take it on trust -- which is the difference between a calculator you can use for something that matters and one you cannot.

Everything runs inside your browser. What you type is never uploaded, never logged and never stored, and the calculator keeps working if your connection drops after the page has loaded.

• Shows network address, usable host range, usable hosts, subnet mask and broadcast address together, so you are not running it twice
• A results table you can read across
• Updates as you type, with no button to press and no page reload
• Copy the result in one click
• Free, no sign-up, and it runs entirely in your browser

The subnet calculator is used by writers, developers, students, marketers and anyone else who needs the job done once without installing software. Common cases include:

• Splitting an office network into subnets of the right size
• Checking which network an address belongs to
• Finding the usable host range for a DHCP scope
• Converting between a prefix length and a dotted mask