Classless Inter-Domain Routing (CIDR): The Internet Address Assignment and Aggregation Plan
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V. Fuller, T. Li · 2006-08
Abstract
RFC 4632 specifies Classless Inter-Domain Routing (CIDR), which replaced the original classful IPv4 addressing scheme with a flexible prefix-length notation. CIDR allows arbitrary-length network prefixes (e.g., /22, /25) and enables route aggregation that dramatically reduces the size of internet routing tables.
Why This RFC Matters
CIDR was one of the most important operational interventions in internet history. By 1993, classful routing — where every network was either a /8, /16, or /24 — was causing the global BGP routing table to explode at a rate that threatened router memory limits. CIDR, introduced in RFC 1519 and updated here in RFC 4632, allowed ISPs to aggregate contiguous blocks into single route announcements, reducing routing table growth by roughly an order of magnitude. The slash-notation (e.g., 192.168.1.0/24) that everyone who configures a network uses today is CIDR notation, making RFC 4632 one of the most practically influential RFCs ever written.