CIDR Calculator

CIDR (Classless Inter-Domain Routing) notation describes an IP network as an address followed by a slash and a prefix length, such as 192.168.1.0/24. The prefix length (24) is the number of leading 1-bits in the subnet mask — so /24 means the first 24 bits identify the network and the remaining 8 bits identify hosts. This replaced the rigid classful A/B/C system in 1993 and is the standard way IPv4 and IPv6 networks are expressed today.

Each /X notation fixes the first X bits as the network portion and leaves the remaining 32−X bits for hosts. /8 has a 255.0.0.0 mask and 16.7 million addresses; /16 has a 255.255.0.0 mask and 65,536 addresses; /24 has a 255.255.255.0 mask and 256 addresses; /30 has 4 addresses (2 usable); /31 is a point-to-point link; and /32 is a single address. Lower prefix numbers cover larger blocks, higher numbers cover smaller blocks.

A /24 has 256 total addresses (2^(32−24)). Two are reserved — the network address (e.g. 192.168.1.0) and the broadcast address (192.168.1.255) — leaving 254 usable hosts. The rule of thumb is usable hosts = 2^(32−prefix) − 2 for prefixes up to /30. The exception is /31 (point-to-point), where both addresses are usable, and /32, which is a single address.

Count the consecutive 1-bits from the left of the mask. 255.255.255.0 in binary is 11111111.11111111.11111111.00000000 — twenty-four 1-bits, so the prefix is /24. Likewise 255.255.255.192 has 26 ones and is /26. This tool accepts either form: type 255.255.255.192 as the mask and the prefix is computed for you.

The network address is the base of the block — all host bits are 0. The broadcast address is the top of the block — all host bits are 1; it is used to reach every host on the subnet at once. Everything between them is the host range, and for most subnets the first and last usable addresses are network+1 and broadcast−1. For 192.168.1.0/24: network is 192.168.1.0, broadcast is 192.168.1.255, and usable hosts run 192.168.1.1–192.168.1.254.

The wildcard mask is the bitwise complement of the subnet mask — it is used in ACLs and firewall rules to say which bits must match. For 255.255.255.0 the wildcard is 0.0.0.255; for 255.255.255.192 it is 0.0.0.63. A wildcard bit of 1 means "this bit may vary" and 0 means "this bit must match", so 192.168.1.0 0.0.0.255 matches all addresses in the 192.168.1.x range.

Take the number of extra bits you borrow from the host portion as the exponent. Splitting a /24 into /28 subnets borrows 4 bits, giving 2^4 = 16 subnets of 16 addresses (14 usable) each. In general: subnets = 2^(newPrefix − basePrefix) and addresses per subnet = 2^(32 − newPrefix).

Yes — completely free, no registration, and 100% private. Every calculation runs locally in your browser with plain JavaScript; your IP addresses are never sent to any server. The tool works offline after the page loads, so you can compute subnet masks and IP ranges even on a disconnected machine.