Map a CIDR block to its full IP scope.

A CIDR block like 192.168.1.0/24 describes a contiguous range of IP addresses. The number after the slash — the prefix length — is how many leading bits of the 32-bit address are fixed as the network portion. The remaining bits are the host portion, free to vary, and they define how many addresses the block contains.

Network, broadcast, and the usable range

Setting every host bit to 0 gives the network address. Setting every host bit to 1 gives the broadcast address. In a normal subnet those two are reserved, so the usable host range runs from network+1 to broadcast−1. That's why a /24 holds 256 addresses but only 254 usable hosts.

Two special cases

A /31 has no room for separate network and broadcast addresses, so RFC 3021 defines both as usable — exactly two hosts, ideal for point-to-point links. A /32 is a single host route. This calculator handles both rather than reporting "0 usable hosts."

Subnet mask vs. wildcard mask

The subnet mask is the prefix written as dotted decimal — /24 is 255.255.255.0. The wildcard mask is its bitwise inverse (0.0.0.255) and is what you use in Cisco ACLs and OSPF, where a 1 bit means "don't care."

Prefix reference

Private ranges (RFC 1918)

Three ranges are reserved for private use and won't route on the public internet: 10.0.0.0/8, 172.16.0.0/12, and 192.168.0.0/16. This calculator flags whether your block is private, public, or special-use.