IP address

This article is about IP addresses in general. For the Wikipedia user access level, see Wikipedia:User access levels#Anonymous users.

An Internet Protocol (IP) address is a numerical label that is assigned to devices participating in a computer network utilizing the Internet Protocol for communication between its nodes.[1] An IP address serves two principal functions in networking: host or network interface identification and location addressing. The role of the IP address has also been characterized as follows: "A name indicates what we seek. An address indicates where it is. A route indicates how to get there."[2]

The original designers of TCP/IP defined an IP address as a 32-bit number[1] and this system, known as Internet Protocol Version 4 or IPv4, is still in use today. However, due to the enormous growth of the Internet and the resulting depletion of available addresses, a new addressing system (IPv6), using 128 bits for the address, was developed in 1995[3] and last standardized by RFC 2460 in 1998.[4] Although IP addresses are stored as binary numbers, they are usually displayed in human-readable notations, such as 208.77.188.166 (for IPv4), and 2001:db8:0:1234:0:567:1:1 (for IPv6).

The Internet Protocol also has the task of routing data packets between networks, and IP addresses specify the locations of the source and destination nodes in the topology of the routing system. For this purpose, some of the bits in an IP address are used to designate a subnetwork. The number of these bits is indicated in CIDR notation, appended to the IP address, e.g., 208.77.188.166/24.

With the development of private networks and the threat of IPv4 address exhaustion, a group of private address spaces was set aside by RFC 1918. These private addresses may be used by anyone on private networks. They are often used with network address translators to connect to the global public Internet.

The Internet Assigned Numbers Authority (IANA) manages the IP address space allocations globally. IANA works in cooperation with five Regional Internet Registries (RIRs) to allocate IP address blocks to Local Internet Registries (Internet service providers) and other entities.

IP versions

Two versions of the Internet Protocol (IP) are currently in use (see IP version history for details), IP Version 4 and IP Version 6. Each version defines an IP address differently. Because of its prevalence, the generic term IP address typically still refers to the addresses defined by IPv4.

IPv4 subnetting

In the early stages of development of the Internet Protocol,[1] network administrators interpreted an IP address in two parts, network number portion and host number portion. The highest order octet (most significant eight bits) in an address was designated the network number and the rest of the bits were called the rest field or host identifier and were used for host numbering within a network. This method soon proved inadequate as additional networks developed that were independent from the existing networks already designated by a network number. In 1981, the Internet addressing specification was revised with the introduction of classful network architecture.[2]

Classful network design allowed for a larger number of individual network assignments. The first three bits of the most significant octet of an IP address was defined as the class of the address. Three classes (A, B, and C) were defined for universal unicast addressing. Depending on the class derived, the network identification was based on octet boundary segments of the entire address. Each class used successively additional octets in the network identifier, thus reducing the possible number of hosts in the higher order classes (B and C). The following table gives an overview of this now obsolete system.

Although classful network design was a successful developmental stage, it proved unscalable in the rapid expansion of the Internet and was abandoned when Classless Inter-Domain Routing (CIDR) was created for the allocation of IP address blocks and new rules of routing protocol packets using IPv4 addresses. CIDR is based on variable-length subnet masking (VLSM) to allow allocation and routing on arbitrary-length prefixes.

Today, remnants of classful network concepts function only in a limited scope as the default configuration parameters of some network software and hardware components (e.g. netmask), and in the technical jargon used in network administrators' discussions.