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An example IP address is 207.142.131.236. Converting to such numbers from the more human-readable form of domain addresses, such as www.wikipedia.org, is done via the Domain Name System. The process of conversion is known as resolution of the domain name.
The Internet Protocol (IP) knows each logical host interface by a number, the so-called IP address. On any given network, this number must be unique among all the host interfaces that communicate through this network. Users of the Internet are sometimes given a host name in addition to their numerical IP address by their ISP.
The IP addresses of users browsing the world wide web are used to enable communications with the server of the web site. Also, it is usually in the header of email messages one sends. Depending on one's Internet connection the IP address can be the same every time one connects, a static IP address, or different per session (but the first part being the same each time): a dynamic IP address.
Internet addresses are needed not only for unique enumeration of host interfaces, but also for routing purposes, therefore a high fraction of them are always unused or reserved.
In IPv4, the current standard protocol for the Internet, IP addresses consist of 32 bits, which makes for 4,294,967,296 (over 4 US billion) unique host interface addresses in theory. In practice the address space is sparsely populated due to routing issues, so that there is some pressure to extend the address range via IP version 6 (see below).
IPv4 addresses are commonly expressed as a dotted quad, four octets (8 bits) separated by periods. The host known as www.wikipedia.org currently has the number 3482223596, written as 207.142.131.236 due to base-256 conversion: 3482223596 in base-10 equals 207*2563+142*2562+131*2561+236*2560. (Resolving the name "www.wikipedia.org" to its associated number is handled by Domain Name System servers.)
Historically, IPv4 addresses originally had only two parts. A later change increased that to three parts: the network part, the subnetwork part, and the host part, in that order. However, with the advent of CIDR, this is no longer true, and the address can have an arbitrary number of levels of hierarchyA hierarchy (Greek hieros sacred, arkho rule) is a system of ranking and organizing things. Different fields use the word in slightly different ways, but a particular definition (below) captures the core of almost all uses. Originally, "hierarchy" meant ". (Technically, this was already true any time after the advent of subnets, since a site could elect to have more than one level of subnetting inside a network number.)
The actual assignment of an address is not arbitrary. An organization, typically an Internet service provider, requests an assignment of a netblock from a registry such as ARIN (American Registry for Internet Numbers). The network number comprises a range of addresses which the organization is free to allocate as they wish. An organization that has exhausted a significant part of its allocated address space, can request another netblock.
For example, ARIN has allocated the addresses 64.78.200.0 through 64.78.207.255 to Verado, Inc. In turn, Verado has allocated the addresses 64.78.205.0 through 64.78.205.15 to Bomis. Bomis, in turn, has assigned the specific address 64.78.205.6 to the host interface that is named www.wikipedia.com.