Classless Inter-Domain Routing
CIDR
<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}.
(2006-01-26)