What is CIDR?

CIDR (Classless Inter-Domain Routing, pronounced "cider") is an IP addressing scheme introduced in 1993 (RFC 1519) that replaced the rigid classful system. CIDR notation represents an address range using a network/prefix-length format.

For example, 192.0.2.0/24 means: "the 256 IP addresses from 192.0.2.0 to 192.0.2.255". The /24 indicates that the first 24 bits of the address are the fixed network portion; the remaining 8 bits identify hosts within that network.

Common CIDR sizes:

• /8 — 16,777,216 IPs (a classical "Class A" block)
• /16 — 65,536 IPs (a classical "Class B")
• /24 — 256 IPs (a classical "Class C")
• /32 — 1 IP (a single host)

CIDR enabled the Internet to grow past the wasteful classful scheme by allowing arbitrary-sized blocks. Today every BGP announcement uses CIDR, every RIR allocation uses CIDR, and firewall rules use CIDR.

Related: IP address, subnet mask, BGP.