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Definition: IPv4 addressing

The format of an IP address in the traditional 32-bit version of the IP protocol. For the forseeable future, IPv4 will co-exist with the newer IPv6 version (see IPv6). IPv4 uses a "dotted decimal" address that comprises four sets of numbers separated by decimal points; for example,

The Domain Name System (DNS) exists so that users do not have to enter numbers. For example, the DNS converts the computerlanguage.com domain to its numeric IP address. However, the control panels in network devices such as routers and gateways are actually addressed by entering their dotted decimal IP number into a browser. See DNS.

Class A, B and C
While the newer 128-bit IPv6 format offers an unlimited number of unique IP addresses (see IPv6), a variable format was adopted to expand the IPv4 range (for details, see CIDR). In summary, addresses are split between network and host. The host (client, server, etc.) is further divided into subnetworks (see subnet mask). All IP addresses are either Class A, B or C. The small number of Class A addresses are reserved for government and huge companies, while more than two million Class C addresses are assigned to ISPs. See private IP address and IP.


         Maximum     Maximum      Bits
 Class   Networks    Hosts      Net/Host

   A         127    16,777,214    7/24
   B      16,383        65,534   14/16
   C   2,097,151           254   21/8

Networks, Subnets and Hosts
An IPv4 address is divided between networks and hosts. The host bits are further divided between subnets and hosts. See subnet mask and CIDR.