Classless Inter-Domain Routing

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(Redirected from CIDR)

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:

  • More efficient use of increasingly scarce IPv4 addresses.
  • Greater use of hierarchy in address assignments (prefix aggregation), lowering the overhead of the Internet-wide routing.
Contents

Background

IP addresses are separated into two parts: the network address (which identifies a whole network or subnet), and the host address (which identifies a particular machine's connection or interface to that network). This division is used to control how traffic was routed in and among IP networks.

Historically, the IP address space was divided into three main 'classes of 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 bitmask 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, defining the length of the subnet mask, which determines the size of the network.

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

  • 192.168.0.0 /24 represents the 256 IPv4 addresses 192.168.0.0 through 192.168.0.255 inclusive, with 192.168.0.255 being the broadcast address for the network.
  • 192.168.0.0 /22 represents the 1024 IPv4 addresses 192.168.0.0 through 192.168.3.255 inclusive, with 192.168.3.255 being the broadcast address for the network.
  • 2002:C0A8::/48 represents the IPv6 addresses 2002:C0A8:0:0:0:0:0:0 through 2002:C0A8:0:FFFF:FFFF:FFFF:FFFF:FFFF, inclusive.

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

  • 192.168.0.0 /24 could be written 192.168.0.0 255.255.255.0
  • 192.168.0.0 /22 could be written 192.168.0.0 255.255.252.0

External links

pt:CIDR

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