Editing IPv4 (section)

==={{anchor|Allocation}}Allocation===
In the original design of IPv4, an IP address was divided into two parts: the network identifier was the most significant octet of the address, and the host identifier was the rest of the address. The latter was also called the ''rest field''. This structure permitted a maximum of 256 network identifiers, which was quickly found to be inadequate.

To overcome this limit, the most-significant address octet was redefined in 1981 to create ''network classes'', in a system which later became known as ''classful'' networking. The revised system defined five classes. Classes A, B, and C had different bit lengths for network identification. The rest of the address was used as previously to identify a host within a network. Because of the different sizes of fields in different classes, each network class had a different capacity for addressing hosts. In addition to the three classes for addressing hosts, Class D was defined for [[multicast]] addressing and Class E was reserved for future applications.

Dividing existing classful networks into subnets began in 1985 with the publication of {{IETF RFC|950}}. This division was made more flexible with the introduction of variable-length subnet masks (VLSM) in {{IETF RFC|1109}} in 1987. In 1993, based on this work, {{IETF RFC|1517}} introduced [[Classless Inter-Domain Routing]] (CIDR),<ref>{{cite web|url=http://www.3com.com/other/pdfs/infra/corpinfo/en_US/501302.pdf|archive-url=https://web.archive.org/web/20010616012053/http://www.3com.com/other/pdfs/infra/corpinfo/en_US/501302.pdf|archive-date=June 16, 2001|title=Understanding IP Addressing: Everything You Ever Wanted To Know|publisher=3Com|url-status=dead}}</ref> which expressed the number of bits (from the [[Most significant bit|most significant]]) as, for instance, {{IPaddr|/24}}, and the class-based scheme was dubbed ''classful'', by contrast. CIDR was designed to permit repartitioning of any address space so that smaller or larger blocks of addresses could be allocated to users. The hierarchical structure created by CIDR is managed by the [[Internet Assigned Numbers Authority]] (IANA) and the [[regional Internet registries]] (RIRs). Each RIR maintains a publicly searchable [[WHOIS]] database that provides information about IP address assignments.