An IP address is a unique numeric identifier for a computer or other device on a TCP/IP network. TCP/IP (transmission control protocol/Internet protocol) is the set of protocols (i.e., agreed upon formats) that is used for the Internet as well as for most LANs (local area networks) and other computer networks.
In IPv4, the current standard protocol for the Internet, each IP address consists of 32 bits. They are expressed as four sets of numbers, each between 0 and 255, which are separated by periods. Examples are 126.96.36.199 and 127.0.0.1; the latter is the so-called loopback address, which returns messages to the same computer that sent them and is used for testing purposes and by some applications.
32 bits allows the creation of more than four billion (exactly 4,294,967,296) unique addresses. However, in practice, the address space is sparsely populated due to routing issues. Routing, which is usually performed by a dedicated device called a router, is the process of moving packets (i.e., the most basic unit of data transmission) from source to destination. Thus there is some pressure to extend the address range though the use of IPv6, which is the next-generation Internet protocol.
IPv4 addresses originally had only two parts, but a later change increased that to three: network, the subnetwork and host, in that order. However, the introduction of CIDR (classless inter-domain routing) now allows addresses to have any number of levels of hierarchy.
Within an isolated network, IP addresses can be assigned at random as long as each one is unique. However, for computers connected to the Internet, it is necessary to use registered IP addresses in order to avoid duplicates.
A static IP address is an IP address for a computer or other device that remains the same every time the device is connected to the network and does not change unless it is changed manually. A dynamic IP address is one that changes every time a device is connected to the network and which is assigned by the dynamic host configuration protocol (DHCP). The dynamic assignment of IP addresses can eliminate the need for system administrators to assign them manually and is a way to make more efficient use of the limited number of IP addresses available to individual ISPs (Internet service providers), businesses and other organizations. Users of dial-up connections to the Internet generally receive dynamically generated IP addresses, whereas users of DSL and cable connections typically are assigned one or more static IP addresses.
IP address assignments are made by registry organizations, such as ARIN (American Registry for Internet Numbers), in response to requests from ISPs and other organizations for a netblock (a range of consecutive IP addresses). If an organization has exhausted a substantial part of its allocated netblock, it can request another.
IPv6, which is expected to be slowly phased in over the next five to 15 years, features a 128-bit addressing scheme that allows for a vastly increased number of unique addresses (18,445,618,199,572,250,625). This will facilitate creation of the so-called Internet of things, which will allow a vast array of objects in addition to computers to be connected to the Internet. IPv5 exists only on an experimental basis and will not be deployed for general use.
Created September 23, 2005. Updated July 12, 2006.