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 192.168.100.1 (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. 192.168.100.1/16) 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.
IPv4 addresses are usually represented in dot-decimal notation (four numbers, each ranging from 0 to 255, separated by dots, e.g. 126.96.36.199). 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 255.255.0.0 in dot-decimal would be FF.FF.00.00 in hexadecimal.)
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)|
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).
|IANA Reserved Private Network Ranges||Start of range||End of range||Total addresses|
|24-bit Block (/8 prefix, 1 x A)||10.0.0.0||10.255.255.255||16,777,216|
|20-bit Block (/12 prefix, 16 x B)||172.16.0.0||172.31.255.255||1,048,576|
|16-bit Block (/16 prefix, 256 x C)||192.168.0.0||192.168.255.255||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 192.168.0.0 - 192.168.0.255 (192.168.0.0/24).
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.
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.
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 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 subnetwork.
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 169.254.0.0/16 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.
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.
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.