Classless Inter-Domain Routing: Difference between revisions

Revision as of 22:28, 8 November 2005

Revision as of 00:56, 9 November 2005

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==CIDR and masks==

A [[Subnetwork|subnet mask]] is a [[Mask (computing)|bitmask]] whichthat shows whereencodes the networkprefix addresslength endsin anda theform host address begins. CIDR uses '''variable length subnet masks''' ('''VLSM''')similiar to allocatean IP addressesaddress to- subnets32 accordingbits, tostarting individualwith need,a rathernumber thanof some1 generalbits network-wideequal rule. Thusto the network/hostprefix divisionlength, canending occurwith at0 anybits, bitand boundaryencoded in thefour-part addressdotted-decimal format. The processA cansubnet bemask [[recursion|recursive]], with a portion ofencodes the addresssame spaceinformation beingas furthera dividedprefix intolength, evenbut smaller portions, throughpredates the useadvent of masks which cover more bitsCIDR.

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 [[recursion|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.

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==Historical Background==

[[IP address]]es arewere originally separated into two parts: the ''network address'' (which identifiesidentified a whole network or subnet), and the ''host address'' (which identifiesidentified a particular machine's connection or interface to that network). This division iswas used to control how traffic iswas routed in and among [[Internet Protocol|IP]] networks.

Historically, the IP address space was divided into three main '[[classful network|classes of network]]', where each class had a fixed size network sizeaddress. The class, and hence the length of the subnetnetwork maskaddress 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 thea prefix length ofor a subnet mask, [[routing|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 table]]s.

As the experimental TCP/IP network expanded into the Internet during the 1980s, the need for more flexible addressing schemes became increasingly apparent. This led to the successive development of [[Subnet|subnetting]], Variable-Length Subnetting, and then CIDR. Because the normalold class distinctions are now ignored, the new system was called '''classless routing'''. This led to the original system being called, by [[back-formation]], '''classful routing'''.

Historically, the IP address space was divided into three main '[[classful network|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|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 table]]s.

==External links==