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IP address

An Internet Protocol (IP) address is a numerical identification (logical address) that is assigned to devices participating in a computer network utilizing the Internet Protocol for communication between its nodes. Although IP addresses are stored as binary numbers, they are usually displayed in human-readable notations, such as (for IPv4), and 2001:db8:0:1234:0:567:1:1 (for IPv6). The role of the IP address has been characterized as follows: "A name indicates what we seek. An address indicates where it is. A route indicates how to get there."

The original designers of TCP/IP defined an IP address as a 32-bit number and this system, now named Internet Protocol Version 4 (IPv4), is still in use today. However, due to the enormous growth of the Internet and the resulting depletion of the address space, a new addressing system (IPv6), using 128 bits for the address, was developed (RFC 1883).

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. (In CIDR notation, the number of bits used for the subnet follows the IP address. E.g. An IP address can be private, for use on a LAN, or public, for use on the Internet or other WAN.

Early specifications intended IP addresses to be uniquely assigned to a particular computer or device. However, it was found that this was not always necessary as private networks developed and address space needed to be conserved (IPv4 address exhaustion). RFC 1918 specifies private address spaces that may be reused by anyone; today, such private networks typically access the Internet through Network Address Translation (NAT). In addition, technologies such as anycast addressing have been developed to allow multiple hosts at the same IP address but in different portions of the Internet to service requests by network clients.

The Internet Assigned Numbers Authority (IANA) manages the global IP address space. 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

The Internet Protocol (IP) has two versions currently in use (see IP version history for details). Each version has its own definition of an IP address. Because of its prevalence, "IP address" typically refers to those defined by IPv4.

IP version 4 addresses

IPv4 uses 32-bit (4-byte) addresses, which limits the address space to 4,294,967,296 (232) possible unique addresses. However, IPv4 reserves some addresses for special purposes such as private networks (~18 million addresses) or multicast addresses (~270 million addresses). This reduces the number of addresses that can be allocated as public Internet addresses, and as the number of addresses available is consumed, an IPv4 address shortage appears to be inevitable in the long run. This limitation has helped stimulate the push towards IPv6, which is currently in the early stages of deployment and is currently the only contender to replace IPv4.

IPv4 addresses are usually represented in dot-decimal notation (four numbers, each ranging from 0 to 255, separated by dots, e.g. Each part represents 8 bits of the address, and is therefore called an octet. It is possible, although less common, to write IPv4 addresses in binary or hexadecimal. When converting, each octet is treated as a separate number. (So in dot-decimal would be FF.FF.00.00 in hexadecimal.)

IPv4 address networks

In the early stages of development of the Internet protocol, network administrators interpreted IP addresses as structures of network numbers and host numbers, with the highest order octet (first eight bits) of an IP address designating the "network number", and the rest of the bits (called the "rest" field) used for host numbering within a network. This method soon proved inadequate as local area networks developed that were not part of the larger networks already designated by a network number. In 1981 IP protocol specification was revised with the introduction of the classful network architecture.

Classful network design allowed for a larger number of individual allocations. The first three bits of the most significant octet of an IP address came to imply the "class" of the address instead of just the network number and, depending on the class derived, the network designation was based on octet boundary segments of the entire address. The following table gives an overview of this system.

Class First octet in binary Range of first octet Network ID Host ID Possible number of networks Possible number of hosts
A 0XXXXXXX 0 - 127 a b.c.d 128 = (27) 16,777,214 = (224 - 2)
B 10XXXXXX 128 - 191 a.b c.d 16,384 = (214) 65,534 = (216 - 2)
C 110XXXXX 192 - 223 a.b.c d 2,097,152 = (221) 254 = (28 - 2)
For details on design and use see 'subnetwork' and 'classful network'.

Although a successful developmental stage, classful network design proved unscalable in the rapid expansion of the Internet and was abandoned in 1993 when Classless Inter-Domain Routing (CIDR) was introduced (RFC 1517, RFC 1518, RFC 1519) to define a new concept of allocation of IP address blocks and new methods of routing protocol packets using IPv4 addresses. CIDR is based on variable-length subnet masking (VLSM) to allow allocation 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).

IPv4 private addresses

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. Three ranges of IPv4 addresses for private networks, one range for each class (A, B, C), were reserved in RFC 1918. These addresses are not routed on the Internet, and thus need not be coordinated with an IP address registry.

IANA Reserved Private Network Ranges Start of range End of range Total addresses
24-bit Block (/8 prefix, 1 x A) 16,777,216
20-bit Block (/12 prefix, 16 x B) 1,048,576
16-bit Block (/16 prefix, 256 x C) 65,536

Any user may use any block. Typically, a network administrator will divide a block into subnets; for example, many home routers automatically use a default address range of - (

IPv4 address depletion

The IP version 4 address space is rapidly nearing exhaustion of available, officially assignable address blocks.

IP version 6 addresses

The designers of IPv6, the next generation of the Internet Protocol, aimed to replace IPv4 on the Internet. Addresses are 128 bits (16 bytes) wide, which, even with a generous assignment of network blocks, will more than suffice for the foreseeable future. The new address space provides a maximum of 2128, or about 3.403 × 1038 unique addresses. The utilization of this large address space is designed in a fashion that provides more efficient route aggregation across the network worldwide.

Example of an IPv6 address: 2001:0db8:85a3:08d3:1319:8a2e:0370:7334

Writing for Technology Review in 2004, Simson Garfinkel calculated "roughly 5,000 addresses for every square micrometer of the Earth's surface". This enormous magnitude of available IP addresses will be sufficiently large for the indefinite future, even though mobile phones, cars and all types of personal devices are coming to rely on the Internet for everyday purposes.

The above statement, however, involves a common misconception about the IPv6 architecture. Its designers did not intend its large address-space to provide unique addresses for every possible point. Rather, the addressing architecture is such that it allows large blocks to be assigned for specific purposes and, where appropriate, to be aggregated for providing efficient routing. With a large address space, there is not the need to have complex address conservation methods as used in classless inter-domain routing (CIDR).

Windows Vista, Apple Computer's Mac OS X, all modern Linux distributions, and an increasing range of other operating systems include native support for the protocol, but it is not yet widely deployed in other devices.

IPv6 private addresses

Just as IPv4 reserves addresses for private or internal networks, there are blocks of addresses set aside in IPv6 for private addresses. In IPv6, these are referred to as unique local addresses (ULA). RFC 4193 sets aside the routing prefix fc00::/7 for this block. The addresses include a 40-bit pseudorandom number that minimizes the risk of address collisions if sites merge or packets are misrouted.

Early designs (RFC 3513) used a different block for this purpose (fec0::), dubbed site-local addresses. However, the definition of what constituted "sites" remained unclear, and the poorly defined address structure created ambiguities for routing. The address range specification was abandoned and must no longer be used in new systems.

Addresses starting with fe80: — called link-local addresses — are routable only in the local link area. The addresses are assigned automatically by the operating system's IP layer for each network interface. This provides instant network connectivity for any IPv6 host and means that if several hosts connect to a common hub or switch, they have an instant communication path via their link-local IPv6 address.

None of the private address prefixes may be routed in the public Internet.

IP address subnetworks

The technique of subnetting can operate in both IPv4 and IPv6 networks. The IP address is divided into two parts: the network address and the host identifier. The subnet mask (in IPv4 only) or the CIDR prefix determine how the IP address is divided into network and host parts.

A subnet mask is only used for IPv4. Both IP version however use use the CIDR notation. In this, the IP address is followed by a slash and the number of bits used to for the network part, also called the routing prefix. For example, an IP address and its subnet mask may be and, respectively. The CIDR notation for the same IP address and subnet is, because the first 24 bits of the IP address indicate the subnetwork.

Static and dynamic IP addresses

When someone manually configures a computer to use the same IP address each time it powers up, this is known as a Static IP address. In contrast, in situations when the computer's IP address is assigned automatically, it is known as a Dynamic IP address.

Method of assignment

Static IP addresses get manually assigned to a computer by an administrator. The exact procedure varies according to platform. This contrasts with dynamic IP addresses, which are assigned either randomly (by the computer itself, as in Zeroconf), or assigned by a server using Dynamic Host Configuration Protocol (DHCP). Even though IP addresses assigned using DHCP may stay the same for long periods of time, they can generally change. In some cases, a network administrator may implement dynamically assigned static IP addresses. In this case, a DHCP server is used, but it is specifically configured to always assign the same IP address to a particular computer, and never to assign that IP address to another computer. This allows static IP addresses to be configured in one place, without having to specifically configure each computer on the network in a different way.

In the absence of both an administrator (to assign a static IP address) and a DHCP server, the operating system may assign itself an IP address using state-less autoconfiguration methods, such as Zeroconf. These IP addresses, known as link-local addresses, default to the address range in IPv4.

In IPv6, every interface, whether using static or dynamic address assignments, also receives a local-link address automatically in the fe80::/64 subnet.

Uses of dynamic addressing

Dynamic IP addresses are most frequently assigned on LANs and broadband networks by Dynamic Host Configuration Protocol (DHCP) servers. They are used because it avoids the administrative burden of assigning specific static addresses to each device on a network. It also allows many devices to share limited address space on a network if only some of them will be online at a particular time. In most current desktop operating systems, dynamic IP configuration is enabled by default so that a user does not need to manually enter any settings to connect to a network with a DHCP server. DHCP is not the only technology used to assigning dynamic IP addresses. Dialup and some broadband networks use dynamic address features of the Point-to-Point Protocol.

Sticky dynamic IP address

A sticky dynamic IP address or sticky IP is a term created by cable and DSL users to describe a dynamically assigned IP address that does not change often. This is however an informal term as a sticky IP does not differ in any way from other dynamic IP address.

It is true that IP addresses do not change often for cable or DSL users; however they use standard DHCP process. Since the modems are often online for extend periods of time, the leases on the IP addresses keep on being renewed and therefore not being changed.

Should the modem be turned off, a new IP address will most likely be assigned when modem the is turned back on as a different host on the network may have been assigned the old IP address. IP address changes may also be triggered by resetting the DHCP server configuration; therefore causing the modem to receive a new IP address.

Some users take advantage of stickiness to create websites on the cheap, since a static IP address is usually more expensive. And some ISPs counter this by forcibly re-assigning IP addresses every 24 hours or so.

Uses of static addressing

Some infrastructure situations have to use static addressing, such as when finding the Domain Name Service directory host that will translate domain names to IP addresses. Static addresses are also convenient, but not absolutely necessary, to locate servers inside an enterprise. An address obtained from a DNS server comes with a time to live, or caching time, after which it should be looked up to confirm that it has not changed. Even static IP addresses do change as a result of network administration (RFC 2072)

Modifications to IP addressing

IP blocking and firewalls

Firewalls are common on today's Internet. For increased network security, they control access to private networks based on the public IP of the client. Whether using a blacklist or a whitelist, the IP address that is blocked is the perceived public IP address of the client, meaning that if the client is using a proxy server or NAT, blocking one IP address might block many individual people.

IP address translation

Multiple client devices can appear to share IP addresses: either because they are part of a shared hosting web server environment or because an IPv4 network address translator (NAT) or proxy server acts as an intermediary agent on behalf of its customers, in which case the real originating IP addresses might be hidden from the server receiving a request. A common practice is to have a NAT hide a large number of IP addresses in a private network. Only the "outside" interface(s) of the NAT need to have Internet-routable addresses.

Most commonly, the NAT device maps TCP or UDP port numbers on the outside to individual private addresses on the inside. Just as a telephone number may have site-specific extensions, the port numbers are site-specific extensions to an IP address.

In small home networks, NAT functions usually take place in a residential gateway device, typically one marketed as a "router". In this scenario, the computers connected to the router would have 'private' IP addresses and the router would have a 'public' address to communicate with the Internet. This type of router allows several computers to share one public IP address.

See also


External links

  • — including sites for identifying one's IP address


  • IPv4 addresses: RFC 791, RFC 1519, RFC 1918, RFC 2071, RFC 2072
  • IPv6 addresses: RFC 4291, RFC 4192

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