Editing Classless Inter-Domain Routing (section)

==Background==
Each IP address consists of a network prefix followed by a [[host (network)|host]] identifier. In the [[classful network]] architecture of [[IPv4]], the three most significant bits of the 32-bit IP address defined the size of the network prefix for unicast networking, and determined the network class A, B, or C.<ref name=rfc943>{{cite IETF |rfc=943 |title=Assigned Numbers |editor1=J. Reynolds |editor2=J. Postel |date=April 1985}}</ref>

{| class="wikitable"
!Class
!Most-significant bits
!Network prefix size (bits)
!Host identifier size (bits)
!Address range
|-
|A
|0
|8
|24
|0.0.0.0–127.255.255.255
|-
|B
|10
|16
|16
|128.0.0.0–191.255.255.255
|-
|C
|110
|24
|8
|192.0.0.0–223.255.255.255
|-
|D<sup>(multicast)</sup><br/>E<sup>(reserved)</sup>
|1110<br/>1111
|–
|–
|224.0.0.0–255.255.255.255
|}

The advantage of this system is that the network prefix could be determined for any IP address without any further information. The disadvantage is that networks were usually too big or too small for most organizations to use, because only three sizes were available. The smallest allocation and routing block contained 2<sup>8</sup> = 256 addresses, larger than necessary for personal or department networks, but too small for most enterprises. The next larger block contained 2<sup>16</sup> = {{gaps|65|536}} addresses, too large to be used efficiently even by large organizations. But for network users who needed more than {{gaps|65|536}} addresses, the only other size (2<sup>24</sup>) provided far too many, more than 16 million. This led to inefficiencies in address use as well as inefficiencies in routing, because it required a large number of allocated class-C networks with individual route announcements, being geographically dispersed with little opportunity for [[route aggregation]].

Within a decade after the invention of the [[Domain Name System]] (DNS), the classful network method was found not [[scalable]].<ref name="RFC 1517">{{cite IETF |rfc=1517 |title=Applicability Statement for the Implementation of Classless Inter-Domain Routing (CIDR) |editor=R. Hinden |date=September 1993}}</ref> This led to the development of [[subnet]]ting and CIDR. The formerly meaningful class distinctions based on the most-significant address bits were abandoned and the new system was described as ''classless'', in contrast to the old system, which became known as ''classful''. Routing protocols were revised to carry not just IP addresses, but also their subnet masks. Implementing CIDR required every host and router on the Internet to be reprogrammed in small ways—no small feat at a time when the Internet was entering a period of rapid growth. In 1993, the [[Internet Engineering Task Force]] published a new set of standards, {{IETF RFC|1518}} and {{IETF RFC|1519}}, to define this new principle for allocating IP address blocks and routing IPv4 packets. An updated version, {{IETF RFC|4632}}, was published in 2006.<ref name="RFC 4632">{{cite IETF |rfc=4632 |title=Classless Inter-domain Routing (CIDR): The Internet Address Assignment and Aggregation Plan |author1=V. Fuller |author2=T. Li |date=August 2006}}</ref>

After a period of experimentation with various alternatives, Classless Inter-Domain Routing was based on variable-length subnet masking (VLSM), which allows each network to be divided into subnetworks of various power-of-two sizes, so that each subnetwork can be sized appropriately for local needs. Variable-length subnet masks were mentioned as one alternative in {{IETF RFC|950}}.<ref name="RFC 950 2.1">{{cite IETF |rfc=950 |title=Internet Standard Subnetting Procedure |editor1=J. Mogul |editor2=J. Postel |date=August 1985 |section=2.1}}</ref> Techniques for grouping addresses for common operations were based on the concept of cluster addressing, first proposed by Carl-Herbert Rokitansky.<ref>Carl-Herbert Rokitansky, "Internet Cluster Addressing Scheme and its Application to Public Data Networks", Proc. 9th International Conference on Computer Communication (ICCC' 88), pp. 482–491, Tel Aviv, Israel, October/November 1988</ref><ref>[http://www.ietf.org/mail-archive/web/ietf/current/msg24136.html Cluster Addressing and CIDR] in the mail archives of the IETF</ref>

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