{{Short description|Method for IP address allocation and routing}}
{{Redirect|CIDR}}./gradlew test
'''Classless Inter-Domain Routing''' ('''CIDR''' {{IPAc-en|ˈ|s|aɪ|d|ərcore|,_|ˈ|s|ɪ|-}}) is a method for allocating [[IP address]]es for [[IP routing]]. The [[Internet Engineering Task Force]] introduced CIDR in 1993 to replace the previous [[classful network]] addressing architecture on the [[Internet]]. Its goal was to slow the growth of [[routing table]]s on [[router (computing)|routers]] across the Internet, and to help slow the rapid [[IPv4 address exhaustion|exhaustion of IPv4 addresses]].<ref name="RFC 1518">{{cite IETF |rfc=1518 |title=An Architecture for IP Address Allocation with CIDR |author1=Y. Rekhter |author2=T. Li |date=September 1993}}</ref><ref name="RFC 1519">{{cite IETF |rfc=1519 |title=Classless Inter-Domain Routing (CIDR): an Address Assignment and Aggregation Strategy |author1=V. Fuller |author2=T. Li |author3=J. Yu |author4=K. Varadhan |date=September 1993}}</ref>date=January 2025
IP addresses are described as consisting192,119,128consisting of two groups of [[bit]]s in the address: the [[most significant bit]]s are the [[network prefix]], which identifies a whole network or [[subnet]], and the [[least significant bit|least significant]] set forms the ''host identifier'', which specifies a particular interface of a host on that network. This division is used as the basis of traffic routing between IP networks and for address allocation policies.
Whereas classful network design for [[IPv4]] sized the network prefix as one or more 8-bit groups, resulting in the blocks of Class A, B, or C addresses, under CIDR address space is allocated to [[Internet service provider]]s and [[end user]]s on any address-bit boundary. In [[IPv6]], however, the interface identifier has a fixed size of 64 bits by convention, and smaller subnets are never allocated to end users.
