CIDR

CIDR (Classless Inter-Domain Routing) is a method for representing IP networks and prefix lengths using slash notation, for example 192.168.1.0/24. The number after the slash is the prefix length and indicates how many bits belong to the network portion. CIDR replaced the older class-based system (Class A, B and C) and made arbitrary allocation of address blocks possible, independent of fixed class boundaries.

Because the prefix length can be chosen freely, operators can both divide blocks into smaller subnets (subnetting) and combine several blocks into a single announcement (supernetting / aggregation). Aggregation is what keeps the global BGP routing table manageable instead of growing with every individual network.

Concrete examples: 192.168.1.0/24 covers 256 addresses with 254 usable hosts, while 10.0.0.0/8 covers the entire block 10.0.0.0–10.255.255.255. In IPv6, the same notation gives blocks such as 2001:db8::/32 for an allocation and 2001:db8:abcd::/48 for a site. CIDR is used in IPv4, IPv6, firewall rules, ACLs, static routes and BGP announcements.

CIDR also underpins how addresses flow through the registry system: IANA delegates large blocks (for instance /8 in IPv4) to RIRs, which sub-allocate down to LIRs and end-users with shorter or longer prefixes depending on need. Inside a router, multiple overlapping CIDR entries can match the same destination; longest-prefix match selects the most specific one.

CIDR is the notation system — the prefix length itself is just the number after the slash. A subnet mask such as 255.255.255.0 is the dotted-decimal form of the prefix /24 and expresses exactly the same information in IPv4. CIDR is therefore the broader concept, prefix is the number, and subnet mask is the older IPv4-specific representation of that same number.