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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A scheme has been agreed on where the concept of class for an IP network address is abolished, and instead routing decisions are to be taken based on a variable-length (contiguous) prefix of the IP address, thus the term classless routing.
In addition, both Network Address Translation (NAT) and classless routing, two Router Service Card feature upgrades, help carriers reduce operating costs by enabling network planners to optimize available addresses and meet the needs of their individual networks.
As service providers and their customers seek to maximize capital expenditure return on investment and reduce operational expenditures wherever possible, the Adit 600's new dedicated, in-band 64 Kbps feature management channel, NAT and classless routing capabilities will enable customers to achieve these goals.