CIDR / Subnet Calculator

A CIDR block such as 10.0.0.0/16 describes a contiguous range of IP addresses. The prefix length (the number after the slash) is how many leading bits are fixed as the network. The remaining bits address hosts. A /24 fixes 24 bits, leaving 8 for hosts: 256 addresses, 254 usable (the network and broadcast addresses are reserved).

/31 and /32 are special: per RFC 3021 a /31 has 2 usable addresses (point-to-point links), and a /32 is a single host. Everything here is computed locally with bit math. No request leaves your browser.