Classless Inter-Domain Routing (CIDR)

CIDR is the modern method for allocating IP address blocks and expressing routes with variable-length prefixes, enabling efficient address use and route aggregation across the Internet.

Classless Inter-Domain Routing (CIDR) is a method for representing and allocating Internet Protocol address blocks and for expressing routing prefixes in a compact form. Instead of the old classful A/B/C system, CIDR uses a prefix length written as an address followed by a slash and a number (for example, 10.0.0.0/8 or 192.168.1.0/24). The notation indicates how many leading bits form the network portion of an address.

Key characteristics

CIDR represents networks by a base address and a prefix length. The prefix length counts the number of contiguous one bits in the subnet mask: a /8 corresponds to 255.0.0.0, /16 to 255.255.0.0, /24 to 255.255.255.0, and so on. This approach supports variable-length subnet masking (VLSM), so networks of different sizes can be carved from larger blocks without being limited to fixed classes.

History and purpose

CIDR was introduced in the early 1990s by the Internet engineering community to address two problems: the rapid exhaustion of IPv4 address space caused by fixed classful allocation, and the growth of global routing tables. Its adoption allowed Internet Service Providers and registry authorities to assign address blocks more precisely and enabled route aggregation (sometimes called supernetting), which reduced the number of distinct routes exchanged between routers.

Uses and examples

CIDR notation appears in routing tables, firewall and access-control rules, cloud security groups, and address allocation records. Common examples include private networks such as 10.0.0.0/8 and 192.168.0.0/16. Network operators use CIDR to combine multiple small networks into a single advertised prefix, lowering routing overhead. Tools and documentation often link to details about IP addresses and the Internet Protocol when explaining allocations.

Notable points and distinctions

CIDR differs from classful addressing by removing fixed-size classes and allowing arbitrary prefix lengths from 0 to 32 in IPv4 (and up to 128 in IPv6).
Host capacity for an IPv4 prefix is 2^(32 − prefix) addresses; traditional networking subtracts addresses reserved for network and broadcast, though /31 and /32 have special uses.
CIDR requires contiguous mask bits; noncontiguous masks are not part of standard CIDR practice.

Overall, CIDR remains a foundational feature of modern IP networking: it improves address efficiency, supports hierarchical routing, and is used across both IPv4 and IPv6 addressing schemes.