Classless Inter-Domain Routing, or CIDR, is the scheme that rescued IPv4 addressing from the rigidity of its original class system. The defining document is RFC 1519, “Classless Inter-Domain Routing (CIDR): an Address Assignment and Aggregation Strategy,” written by V. Fuller, T. Li, J. Yu, and K. Varadhan and published in September 1993. It proposed abandoning the fixed Class A, B, and C boundaries in favor of representing routing destinations as a “network and mask pair,” where the mask can fall at any bit position rather than only at the class boundaries.
This is the origin of the now-universal slash notation. A block written as 192.0.2.0/24 means the first 24 bits are the network prefix and the remaining 8 bits identify hosts; a /23 covers twice as many addresses, a /25 half as many. By making the prefix length a free parameter, CIDR let an organization receive an address block sized to its actual need instead of being forced to choose between a Class C of 254 addresses and a Class B of more than 65,000. The waste built into classful allocation simply disappeared.
CIDR addressed three linked crises that RFC 1519 lays out explicitly: the imminent exhaustion of Class B network numbers, the explosive growth of the global routing table, and the longer-term depletion of the IPv4 address space itself. The routing-table problem was the most urgent. The document cites historic data showing routing tables doubling roughly every ten months between 1988 and 1991, and warns that without intervention the table could swell toward tens of thousands of entries within a couple of years as sites consumed Class C networks in bulk.
The cure for table growth was aggregation. Because prefixes are no longer tied to class boundaries, a provider that holds a large contiguous block can advertise it to the rest of the internet as a single route, even though it has subdivided that block among many customers internally. Routers elsewhere see one prefix instead of hundreds. RFC 1519 projected that with CIDR deployment, annual routing-table growth could fall dramatically rather than continuing to double, keeping the table at a manageable size. This depends directly on the longest-prefix-match forwarding rule, which lets a broad aggregate route coexist with more specific exceptions when they are needed.
CIDR was later updated and clarified by RFC 4632, but the core idea introduced in 1993 is exactly what the internet still uses. Every modern address allocation is a CIDR prefix, every BGP advertisement carries prefixes rather than classes, and the slash notation is the standard way engineers describe a block of addresses. The class system that RFC 791 had defined in 1981 was effectively retired, not by replacing the 32-bit address, but by changing how its bits are divided between network and host.