Editing Classless Inter-Domain Routing (section)

{{Short description|Method for IP address allocation and routing}}
{{Redirect|CIDR}}

'''Classless Inter-Domain Routing''' ('''CIDR''' {{IPAc-en|ˈ|s|aɪ|d|ər|,_|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>

IP addresses are described as consisting 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.

{{Anchor|VLSM}}CIDR is based on '''variable-length subnet masking''' ('''VLSM'''), in which network prefixes have variable length (as opposed to the fixed-length prefixing of the previous classful network design). The main benefit of this is that it grants finer control of the sizes of subnets allocated to organizations, hence slowing the exhaustion of IPv4 addresses from allocating larger subnets than needed. CIDR gave rise to a new way of writing IP addresses known as CIDR notation, in which an IP address is followed by a suffix indicating the number of bits of the prefix. Some examples of CIDR notation are the addresses {{IPaddr|192.0.2.0|24}} for IPv4 and {{IPaddr|2001:db8::|32}} for IPv6. Blocks of addresses having contiguous prefixes may be aggregated as [[supernet]]s, reducing the number of entries in the global routing table.