Classless Inter-Domain Routing (CIDR), introduced starting in 1993, is the latest refinement to the way IP addresses are interpreted. It replaced the previous generation of IP address syntax, classful networks. It allowed increased flexibility when dividing ranges of IP addresses into separate networks. It thereby promoted:

=Background=

networks.

Historically, the IP address space was divided into three main classful network , where each class had a fixed network size. The class, and hence the length of the subnet mask and the number of hosts on the network, could always be determined from the most significant bits of the IP address. Without any other way of specifying the length of a subnet mask, routing protocols necessarily used the class of the IP address specified in route advertisements to determine the size of the routing prefixes to be set up in the routing tables.

=CIDR and masks=

A subnet mask is a Mask (computing) which shows where the network address ends and the host address begins. CIDR uses variable length subnet masks (VLSM) to allocate IP addresses to subnets according to individual need, rather than some general network-wide rule. Thus the network/host division can occur at any bit boundary in the address. The process can be recursive, with a portion of the address space being further divided into even smaller portions, through the use of masks which cover more bits.

Because the normal class distinctions are ignored, the new system was called classless routing. This led to the original system being called, by back-formation, classful routing.

CIDR/VLSM network addresses are now used throughout the public Internet, although they are also used elsewhere, particularly in large private networks. An average desktop LAN user generally does not see them in practice, as their LAN network is usually numbered using special private RFC 1918 addresses.

= Prefix aggregation =

Another benefit of CIDR is the possibility of routing prefix aggregation. For example, sixteen contiguous /24 networks could now be aggregated together, and advertised to the outside world as a single /20 route (if the first 20 bits of their network addresses match). Two contiguous /20s could then be aggregated to a /19, and so forth. This allowed a significant reduction in the number of routes that had to be advertised over the Internet, preventing routing table explosion from overwhelming routers, and stopping the Internet from expanding further.

=CIDR notation=

The standard notation for a CIDR address range begins with the network address (padded on the right with the appropriate number of zero-valued bits - up to 4 Octets for IPv4, and up to 8 16-bit hexadecimal fields for IPv6). This is followed by a / character and a prefix length, in bits, specifying the number of 1 s in the subnet mask, which determines the size of the network part of an IP address.

For example (a more complete IPv4 subnetting reference table is available):

For IPv4, an alternative representation uses the network address followed by the network s subnet mask, written in dotted decimal form:

The number of hosts per subnet defined by the mask can be calculated as 2(32-mask). For example, a mask of /29 gives: 2(32-29) = 23 = 8 hosts

=External links=