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From Wikipedia, the free encyclopedia

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.^[1] 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 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."^[2]

The original designers of TCP/IP defined an IP address as a 32-bit number^[1] 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. 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.

Contents 1 IP versions 1.1 IP version 4 addresses 1.1.1 IPv4 networks 1.1.2 IPv4 private addresses 1.2 IPv4 address depletion 1.3 IP version 6 addresses 1.3.1 IPv6 private addresses 2 IP subnetworks 3 Static and dynamic IP addresses 3.1 Method of assignment 3.2 Uses of dynamic addressing 3.2.1 Sticky dynamic IP address 3.3 Address autoconfiguration 3.4 Uses of static addressing 4 Modifications to IP addressing 4.1 IP blocking and firewalls 4.2 IP address translation 5 See also 6 References 7 External links 7.1 RFCs

[edit] 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, the generic term IP address typically still refers to the addresses defined by IPv4.

[edit] 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 offering 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. 208.77.188.166). Each part represents 8 bits of the address, and is therefore called an octet. In less common cases of technical writing, IPv4 addresses may be presented in hexadecimal, octal, or binary representations. When converting, each octet is usually treated as a separate number.

[edit] IPv4 networks

In the early stages of development of the Internet protocol,^[1] network administrators interpreted an IP address as a structure of network number and host number. The highest order octet (most significant eight bits) were designated the network number, and the rest of the bits were called the rest field or host identifier and 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, the Internet addressing specification was revised with the introduction of classful network architecture. ^[2]

Classful network design allowed for a larger number of individual assignments. The first three bits of the most significant octet of an IP address was defined as 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)

The articles 'subnetwork' and 'classful network' explain the details of this design.

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) 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.

[edit] IPv4 private addresses

Early network design, when global end-to-end connectivity was envisioned for all Internet hosts, intended that IP addresses 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).

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 their use need not be coordinated with an IP address registry.

Today, such private networks typically connect to the Internet through Network Address Translation (NAT).

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).

[edit] IPv4 address depletion

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

[edit] IP version 6 addresses

The rapid exhaustion of IPv4 address space, despite conservation techniques, prompted the Internet Engineering Task Force (IETF) to explore new technologies to expand the Internet's addressing capability. The permanent solution was deemed to be a redesign of the Internet Protocol itself. This next generation of the Internet Protocol, aimed to replace IPv4 on the Internet, was eventually named Internet Protocol Version 6 (IPv6)^[3] The address size was increased from 32 to 128 bits (16 bytes), which, even with a generous assignment of network blocks, is deemed sufficient for the foreseeable future. Mathematically, the new address space provides the potential for a maximum of 2¹²⁸, or about 3.403 × 10³⁸ unique addresses.

The new design is not based on the goal to provide a sufficient quantity of addresses alone, but rather to allow efficient aggregation of subnet routing prefixes to occur at routing nodes. As a result, routing table sizes are smaller, and the smallest possible individual allocation is a subnet for 2⁶⁴ hosts, which is the size of the square of the size of the entire IPv4 Internet. At these levels, actual address utilization rates will be small on any IPv6 network segment. The new design also provides the opportunity to separate the addressing infrastructure of a network segment--that is the local administration of the segment's available space--from the addressing prefix used to route external traffic for a network. 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 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 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^[update] Linux distributions^[4], and an increasing range of other operating systems include native support for the protocol, but it is not yet widely deployed in other devices.

Example of an IPv6 address:

2001:0db8:85a3:08d3:1319:8a2e:0370:7334

[edit] 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 which is divided into two /8 blocks with different implied policies (cf. IPv6) 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 addressing policy 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 assigned only in the local link area. The addresses are generated usually automatically by the operating system's IP layer for each network interface. This provides instant automatic 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. This feature is used extensively, and invisibly to most users, in the lower layers of IPv6 network administration (cf. Neighbor Discovery Protocol).

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

[edit] IP 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 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 Classless Inter-Domain Routing (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.

[edit] Static and dynamic IP addresses

When a computer is configured 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.

[edit] Method of assignment

Static IP addresses are 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 by the computer interface or host software 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. This allows static IP addresses to be configured centrally, without having to specifically configure each computer on the network in a manual procedure.

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.

[edit] 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.

[edit] Sticky dynamic IP address

This section may contain original research or unverified claims. Please improve the article by adding references. See the talk page for details. (December 2008)

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 addresses.

Even though IP addresses may not change often for cable or DSL users, the addresses are still controlled by the standard DHCP process. Since the modems are often online for extended periods of time, the leases on the IP addresses are commonly renewed, and therefore may not change.

Should the modem be turned off, a new IP address may be assigned when the modem 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.

[edit] Address autoconfiguration

RFC 3330 defines an address block, 169.254.0.0/16, for the special use in link-local addressing for IPv4 networks. In IPv6, every interface, whether using static or dynamic address assignments, also receives a local-link address automatically in the fe80::/10 subnet.

These addresses are only valid on the link, such as a local network segment or point-to-point connection, that a host is connected to. These addresses are not routable and like private addresses cannot be the source or destination of packets traversing the Internet.

When the link-local IPv4 address block was reserved, no standards existed for mechanisms of address autoconfiguration. Filling the void, Microsoft created an implementation that called Automatic Private IP Addressing (APIPA). Due to Microsoft's market power, APIPA has been deployed on millions of machines and has, thus, become a de facto standard in the industry. Many years later, the IETF defined a formal standard for this functionality, RFC 3927, entitled Dynamic Configuration of IPv4 Link-Local Addresses.

[edit] Uses of static addressing

Some infrastructure situations have to use static addressing, such as when finding the Domain Name System 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)

[edit] Modifications to IP addressing

[edit] IP blocking and firewalls

Firewalls are common on today^[update]'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.

[edit] 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^[5].

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.

[edit] See also

[edit] References

• Comer, Douglas (2000). Internetworking with TCP/IP:Principles, Protocols, and Architectures --4th ed.. Upper Saddle River, NJ: Prentice Hall. ISBN 0-13-018380-6. http://www.cs.purdue.edu/homes/dec/netbooks.html. 

[edit] External links

• Articles on CircleID about IP addressing
• How to get a static IP address - clear instructions for all the major platforms
• IP at the Open Directory Project — including sites for identifying one's IP address

• IP-Address Management on LANs — article in Byte magazine – It needs to be checked
• Understanding IP Addressing: Everything You Ever Wanted To Know

[edit] RFCs

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