CIDR Calculator

CIDR (Classless Inter-Domain Routing) is the method used to allocate IP addresses and route traffic on the internet. A CIDR block is written as an IP address followed by a slash and a prefix length: 192.168.1.0/24. The prefix length indicates how many bits of the 32-bit IPv4 address (or 128-bit IPv6 address) are the network portion — the remaining bits identify individual hosts within that network.

The subnet mask is derived from the prefix length: /24 corresponds to 255.255.255.0 (24 binary 1s followed by 8 binary 0s). The network address is the block address with all host bits set to 0. The broadcast address has all host bits set to 1. Usable host addresses are all addresses between network and broadcast — a /24 has 256 total addresses with 254 usable hosts. The first usable address is conventionally assigned to the gateway router.

CIDR replaced the original class-based system (Class A: /8, Class B: /16, Class C: /24) in 1993, enabling more efficient IP address allocation by allowing networks of any prefix length. Common prefix lengths: /32 identifies a single host, /30 has 2 usable addresses (used for point-to-point links), /29 has 6 usable, /28 has 14, /27 has 30, /24 has 254 (the common 'Class C equivalent'), /16 has 65,534, and /8 has over 16 million. The private address ranges defined by RFC 1918 (10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16) are reserved for internal network use and are not routed on the public internet.