CIDR & Subnet Reference

CIDR (Classless Inter-Domain Routing) notation is a compact way to express an IP address and its associated network mask. It is written as an IP address followed by a slash and a prefix length (e.g., 192.168.1.0/24). The prefix length indicates how many leading bits define the network portion, with the remaining bits available for host addresses. CIDR replaced the older classful addressing system (Class A/B/C) to allow more flexible allocation of IP address space.

For IPv4, use the formula 2^(32 - prefix) - 2. You subtract 2 because the first address is reserved as the network address and the last as the broadcast address. For example, a /24 has 2^8 - 2 = 254 usable hosts. Special cases: a /31 has 2 usable addresses (point-to-point links per RFC 3021), and a /32 represents a single host.

Private IP addresses (RFC 1918) are reserved for internal networks and cannot be routed on the public internet. The three private ranges are 10.0.0.0/8, 172.16.0.0/12, and 192.168.0.0/16. Public IP addresses are globally unique and routable on the internet. Devices on private networks use NAT (Network Address Translation) to communicate with the public internet through a shared public IP address.

A wildcard mask is the bitwise inverse of a subnet mask. Where a subnet mask uses 1-bits to mark the network portion, a wildcard mask uses 0-bits for the same purpose. For example, a /24 subnet mask of 255.255.255.0 has a wildcard mask of 0.0.0.255. Wildcard masks are commonly used in Cisco router ACLs and OSPF area definitions. To convert, subtract each subnet mask octet from 255.