Classless Inter-Domain Routing

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Classless Inter-Domain Routing

(networking)
(CIDR) /sid*r/ A technique that summarises a block of Internet addresses in a routing table as an 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.

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

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.

See also RFC 1467, RFC 1518, RFC 1520.
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References in periodicals archive ?
Attackers were found to have contaminated a diverse pool of IP addresses across hundreds of IP prefixes (at least 527 Class C networks, according to Nexusguard findings) with very small-sized junk traffic.
The number of IP prefixes in every AS is calculated from the BGP routing tables collected by Route Views [11] and RIPE RIS [12].
These measures included the exceptional allocation of "Class B", address blocks, the reuse of "Class C" blocks [10], then the abolition of classes in the allocation and routing mechanisms of IP prefixes (CIDR, Classless Internet Domain Routing).