IP addresses:

An Internet Protocol (IP) address is a numerical identification that is assigned to devices participating in a computer network utilizing the Internet Protocol for communication between its nodes. This identification of both the sender and the receiver is included in every datagram transmitted between two (or more) nodes. The address specifies the locations of the source and destination nodes in the topology of the routing system.

The Internet Protocol is the primary network layer that deals with IP addresses. Most other aspect of Internet communications are relegated to other protocols. The role of the IP address has been characterized as follows (e.g., RFC 791, 2.3): "A name indicates what we seek. An address indicates where it is. A route indicates how to get there."

Originally an IP address was defined as a 32-bit number and this is still in use today. This system is now named Internet Protocol Version 4 (IPv4). 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, had to be developed that is now being deployed in many places around in the world, coexisting with the old standard and transmitted over the same hardware and network links.

Although IP addresses are strictly binary entities, humanly more managable representations are used, such as 192.168.100.1 for IPv4, and 2001:db8:0:1234:0:567:1:1 (IPv6), segmenting the full length binary strings into groups of 8 or 16 bits and expressing those as decimal (IPv4) or hexadecimal numbers (IPv6). The Internet Protocol also is responsible for routing data packets on the network. For this purpose IP adresses are often affixed with a designation of the number of bits used for routing purposes. e.g. 192.168.100.1/16 assigns the first 16 bits of the address as routing information (see Classless Inter-Domain Routing).

Originally, an IP address was always uniquely assigned to a particular computer or device. However, it was found 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. 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.

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.

An illustration of an IP address (version 4), in both dot-decimal notation and binary.

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, some are reserved 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 dotted-decimal notation (four numbers, each ranging from 0 to 255, separated by dots, e.g. 147.132.42.18). Each range from 0 to 255 can be represented by 8 bits, 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.)

IPv4 address networks

In the early stages of development of the Internet protocol, IP addresses were interpreted as structures of network numbers and host numbers, with the highest order octet 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 (RFC 791) 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 were used 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.

While being 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 remain in practice only in a limited scope in the default configuration parameters of some network software and hardware components (e.g. netmask).

IPv4 private addresses

Machines not connected to the outside world (e.g. factory machines that communicate with each other via TCP/IP) need not have globally-unique IP addresses. Three ranges of IPv4 addresses for private networks, one per class, were standardized by RFC 1918; these addresses will not be routed, and thus need not be coordinated with any IP address registrars.

Each block is not necessarily one single network, although it is possible. Typically the 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).

An illustration of an IP address (version 6), in hexadecimal and binary.

IP version 6 addresses

IPv6 is a new standard protocol intended to replace IPv4 for the Internet. Addresses are 128 bits (16 bytes) wide, which, even with a generous assignment of netblocks, will more than suffice for the foreseeable future. In theory, there would be exactly 2¹²⁸, or about 3.403 × 10³⁸ unique host interface addresses. Further, this large address space will be sparsely populated, which makes it possible to again encode more routing information into the addresses themselves.

Writing for Technology Review in 2004, Simson Garfinkel wrote that there will exist "roughly 5,000 addresses for every square micrometer of the Earth's surface".^[1] 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 source, however, involves a common misperception about the IPv6 architecture. Its large address space is not intended 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, aggregated for providing 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).

IPv6 private addresses

Just as there are addresses for private, or internal networks in IPv4 (one example being the 192.168.0.0 - 192.168.255.255 range), there are blocks of addresses set aside in IPv6 for private addresses. Addresses starting with FE80: are called link-local addresses and are routable only on your local link area. This means that if several hosts connect to each other through a hub or switch then they would communicate through their link-local IPv6 address.

Early designs specified an address range used for "private" addressing, with prefix FEC0. These are called site-local addresses (SLA) and are routable within a particular site, analogously to IPv4 private addresses. Site-local addresses, however, have been deprecated by the IETF since they create the same problem that does the existing IPv4 private address space (RFC 1918). With that private address space, when two sites need to communicate, they may have duplicate addresses that "combine". In the IPv6 architecture, the preferred method is to have unique addresses in a range not routable on the Internet, issued to organizations (e.g., enterprises).

The preferred alternative to site-local addresses are centrally assigned unique local unicast addresses (ULA). In current proposals, they will start with the prefix FC00.

Neither ULA nor SLA nor link-local address ranges are routable over the internet.

IP address subnetting

subnetting is a technique that can be used in both IPv4 and IPv6 networks. The IP address is divided into two parts: the network address and the host address. The subnet mask determines the IP address is divided into network and host parts.

As an alternative to the subnet mask, CIDR notation can also be used. In CIDR notation, the IP address is followed by a slash and the number of bits used to designate the prefix length, i.e., the network part. For example, a typical 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.

Static and dynamic IP addresses

When a computer is manually 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.

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 randomly (by the computer itself, as in Zeroconf), or arbitrarily 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 still assign itself a dynamic IP address using Zeroconf. These IP addresses are known as link-local addresses. For IPv4, link-local addresses are in the 169.254.0.0/16 address range.

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.

Uses of static addressing

Static addressing is essential in some infrastructure situations, such as finding the Domain Name Service directory host that will translate domain names to numbers (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, however (RFC 2072).

Modifications to IP addressing

IP blocking and firewalls

Firewalls are common on today's Internet. For increased network security, they allow or deny access to their private network 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

IP addresses can appear to be shared by multiple client devices 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^[3].

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

In small home networks, NAT functions are usually performed by 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.

IP addresses

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