CIDR Calculator

CIDR (Classless Inter-Domain Routing) notation represents an IP address and its associated network prefix length. For example, 192.168.1.0/24 means the first 24 bits are the network portion, giving 256 total addresses with 254 usable hosts.

The number of total addresses is 2^(32-prefix). For a /24, that's 2^8 = 256 addresses. Subtract 2 for the network address and broadcast address to get 254 usable hosts. A /30 gives 4 total, 2 usable — ideal for point-to-point links.

A wildcard mask is the inverse of the subnet mask. It's used in access control lists (ACLs) on Cisco routers. For a /24 subnet (mask 255.255.255.0), the wildcard is 0.0.0.255. A 1 bit means 'don't care', a 0 bit means 'must match'.

A /30 has 4 addresses (2 usable hosts + network + broadcast) and is the classic choice for point-to-point links. A /31 has 2 addresses — no broadcast — and is specified in RFC 3021 for point-to-point links to save address space. A /32 is a host route (single address).

RFC 1918 defines three private ranges: 10.0.0.0/8 (16.7M addresses), 172.16.0.0/12 (1M addresses), and 192.168.0.0/16 (65K addresses). These are not routed on the internet and are used for internal networks.