IP address
An Internet Protocol address (IP address) is a numerical label, such as 192.168.1.1 or 2001:0db8:0000:0000:0000:8a2e:0370:7334, that is assigned to a device connected to a computer network that uses the Internet Protocol for communication.[1] IP addresses serve two main functions: network interface identification, and location addressing.
Internet Protocol version 4 (IPv4) was the first standalone specification for the IP address, and has been in use since 1983.[1] IPv4 addresses are defined as a 32-bit number, which later then became too small to provide enough addresses as the internet grew, leading to IPv4 address exhaustion over the 2010s. Its designated successor, IPv6, uses 128 bits for the IP address, giving it a larger address space.[2][3][4] Although IPv6 deployment has been ongoing since the mid-2000s, both IPv4 and IPv6 are still used side-by-side as of 2026[update], with IPv6 recently exceeding 50% globally after 18 years.[5]
IP addresses are usually displayed in a human-readable notation, but systems may use them in various different computer number formats. CIDR notation can also be used to designate how much of the address should be treated as a routing prefix. For example, 192.0.2.1/24 indicates that 24 significant bits of the address are the prefix, with the remaining 8 bits used for host addressing. This is equivalent to the historically used subnet mask (in this case, 255.255.255.0).
The IP address space is managed globally by the Internet Assigned Numbers Authority (IANA) and the five regional Internet registries (RIRs). IANA assigns blocks of IP addresses to the RIRs, which are responsible for distributing them to local Internet registries in their region, such as internet service providers (ISPs) and large institutions. Some addresses are reserved for private networks and are not globally unique.
Within a network, the network administrator assigns an IP address to each device. Such assignments may be on a static (fixed or permanent) or dynamic basis, depending on network practices and software features. Some jurisdictions, such as Canada and the European Union, consider IP addresses to be personal data.[6][7]
Function
An IP address serves two principal functions: it identifies the host, or more specifically, its network interface, and it provides the location of the host in the network, and thus, the capability of establishing a path to that host. Its role has been characterized as follows: "A name indicates what we seek. An address indicates where it is. A route indicates how to get there."[1] The header of each IP packet contains the IP address of the sending host and that of the destination host.
IP versions
Two versions of the Internet Protocol are in common use on the Internet today. The original version of the Internet Protocol that was first deployed in 1983 in the ARPANET, the predecessor of the Internet, is Internet Protocol version 4 (IPv4).
By the early 1990s, the rapid exhaustion of IPv4 address space available for assignment to Internet service providers and end-user organizations prompted the Internet Engineering Task Force (IETF) to explore new technologies to expand addressing capability on the Internet. The result was a redesign of the Internet Protocol, which eventually became known as Internet Protocol Version 6 (IPv6) in 1995.[2][3][4] IPv6 technology was in various testing stages until the mid-2000s, when commercial production deployment commenced.
Today, these two versions of the Internet Protocol are in simultaneous use. Among other technical changes, each version defines the format of addresses differently. Because of the historical prevalence of IPv4, the generic term IP address typically still refers to the addresses defined by IPv4. The gap in version sequence between IPv4 and IPv6 resulted from the assignment of version 5 to the experimental Internet Stream Protocol in 1979, which, however, was never referred to as IPv5.
Versions 1 to 9 were defined, but only 4 and 6 ever gained widespread use. Transmission Control Protocol (the protocol suite later referred to as TCP/IP) versions 1[8] and 2[9] were published in 1974 and 1977. Version 3[10] was defined in 1978, and version 3.1 is the first version in which TCP is separated from IP.[11][12] Version 6 is a synthesis of several suggested versions, version 6 Simple Internet Protocol, version 7 TP/IX: The Next Internet, version 8 P. Internet Protocol, and version 9 TCP and UDP with Bigger Addresses (TUBA).[13][14]
Subnetworks
IP networks may be divided into subnetworks in both IPv4 and IPv6. For this purpose, an IP address is recognized as consisting of two parts: the network prefix in the high-order bits and the remaining bits called the rest field, host identifier, or interface identifier (IPv6), used for host numbering within a network.[1] The subnet mask or CIDR notation determines how the IP address is divided into network and host parts.
The term subnet mask is only used within IPv4. Both IP versions, however, use the CIDR concept and notation. In this, the IP address is followed by a slash and the number (in decimal) of bits used for the network part, also called the routing prefix. For example, an IPv4 address and its subnet mask may be 192.0.2.1 and 255.255.255.0, respectively. The CIDR notation for the same IP address and subnet is 192.0.2.1/24, because the first 24 bits of the IP address indicate the network and subnet.
IPv4 addresses
An IPv4 address has a size of 32 bits, which limits the address space to 4294967296 (232) addresses. Of this number, some addresses are reserved for special purposes such as private networks (≈18 million addresses) and multicast addressing (≈270 million addresses).
IPv4 addresses are usually represented in dot-decimal notation, consisting of four decimal numbers, each ranging from 0 to 255, separated by dots, e.g., 192.0.2.1. Each part represents a group of 8 bits (an octet) of the address.[15] In some cases of technical writing,[specify] IPv4 addresses may be presented in various hexadecimal, octal, or binary representations.
Subnetting history
In the early stages of development of the Internet Protocol, the network number was always the highest-order octet (most significant eight bits). Because this method allowed for only 256 networks, it soon proved inadequate as additional networks developed that were independent of the existing networks already designated by a network number. In 1981, the addressing specification was revised with the introduction of classful network architecture.[1]
Classful network design allowed for a larger number of individual network assignments and fine-grained subnetwork design. The first three bits of the most significant octet of an IP address were 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.
| Class | Leading bits |
Size of network number bit field |
Size of rest bit field |
Number of networks |
Number of addresses per network |
Start address | End address |
|---|---|---|---|---|---|---|---|
| A | 0 | 8 | 24 | 128 (27) | 16777216 (224) | 0.0.0.0 | 127.255.255.255 |
| B | 10 | 16 | 16 | 16384 (214) | 65536 (216) | 128.0.0.0 | 191.255.255.255 |
| C | 110 | 24 | 8 | 2097152 (221) | 256 (28) | 192.0.0.0 | 223.255.255.255 |
Classful network design served its purpose in the startup stage of the Internet, but it lacked scalability in the face of the rapid expansion of networking in the 1990s. The class system of the address space was replaced with Classless Inter-Domain Routing (CIDR) in 1993.[citation needed] CIDR is based on variable-length subnet masking (VLSM) to allow allocation and routing based 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.
Private addresses
Early network design, when global end-to-end connectivity was envisioned for communications with all Internet hosts, intended that IP addresses be globally unique. However, it was found that this was not always necessary as private networks developed and public address space needed to be conserved.
Computers not connected to the Internet, such as factory machines that communicate only with each other via TCP/IP, need not have globally unique IP addresses. Today, such private networks are widely used and typically connect to the Internet with network address translation (NAT), when needed.
Three non-overlapping ranges of IPv4 addresses for private networks are reserved.[16] These addresses are not routed on the Internet, and thus their use need not be coordinated with an IP address registry. Any user may use any of the reserved blocks. Typically, a network administrator will divide a block into subnets; for example, many home routers automatically use a default address range of 192.168.0.0 through 192.168.0.255 (192.168.0.0/24).
| Name | CIDR block | Address range | Number of addresses | Classful description |
|---|---|---|---|---|
| 24-bit block | 10.0.0.0/8 | 10.0.0.0 – 10.255.255.255 | 16777216 | Single Class A |
| 20-bit block | 172.16.0.0/12 | 172.16.0.0 – 172.31.255.255 | 1048576 | Contiguous range of 16 Class B blocks |
| 16-bit block | 192.168.0.0/16 | 192.168.0.0 – 192.168.255.255 | 65536 | Contiguous range of 256 Class C blocks |
IPv6 addresses
In IPv6, the address size was increased from 32 bits in IPv4 to 128 bits, thus providing up to 2128 (approximately 3.403×1038) addresses. This is deemed sufficient for the foreseeable future.
The intent of the new design was not to provide just a sufficient quantity of addresses, but also to redesign routing in the Internet by allowing more efficient aggregation of subnetwork routing prefixes. This resulted in slower growth of routing tables in routers. The smallest possible individual allocation is a subnet for 264 hosts, which is the square of the size of the entire IPv4 Internet. At these levels, actual address utilization ratios will be small on any IPv6 network segment. The new design also provides the opportunity to separate the addressing infrastructure of a network segment, i.e., the local administration of the segment's available space, from the addressing prefix used to route traffic to and from external networks. IPv6 has facilities that automatically change the routing prefix of entire networks, should the global connectivity or the routing policy change, without requiring internal redesign or manual renumbering.
The large number of IPv6 addresses allows large blocks to be assigned for specific purposes and, where appropriate, to be aggregated for efficient routing. With a large address space, there is no need to have complex address conservation methods as used in CIDR.
All modern desktop and enterprise server operating systems include native support for IPv6, but it is not yet widely deployed in other devices, such as residential networking routers, voice over IP (VoIP) and multimedia equipment, and some networking hardware.
Private addresses
Just as IPv4 reserves addresses for private networks, blocks of addresses are set aside in IPv6. In IPv6, these are referred to as unique local addresses (ULAs). The routing prefix fc00::/7 is reserved for this block,[17] which is divided into two /8 blocks with different implied policies. The addresses include a 40-bit