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Classless Inter-Domain Routing (CIDR) is a technique that summarizes a block of Internet addresses in a routing table as an IP address in dotted decimal notation followed by a forward slash and a two-digit decimal number giving the number of leading one bits in the subnet mask. For example, 123.123.123.0/24 specifies a subnet mask of 11111111.11111111.11111111.00000000 (binary), implying the block of addresses 123.123.123.0 through 123.123.123.255.

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According to RFC 1519, CIDR was implemented to distribute Internet address space more efficiently and to provide a mechanism for IP route aggregation. This in turn reduces the number of entries in IP routing tables, enabling faster, more efficient routing, e.g. using routing protocols such as OSPF. CIDR is supported by BGP4.[1]

CIDR is "classless" because it is not limited to the subnet masks specified by IP address classes A, B and C.[1]

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