The designers of TCP/IP defined an IP address as a 32-bit number[1] and this system, known as Internet Protocol Version 4 or IPv4, is still in use today. However, due to the enormous growth of the Internet and the resulting depletion of available addresses, a new addressing system (IPv6), using 128 bits for the address, was developed in 1995[3] and last standardized by RFC 2460 in 1998.[4] 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 Internet Protocol also routes data packets between networks; 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.
As the development of private networks raised the threat of IPv4 address exhaustion, RFC 1918 set aside a group of private address spaces that 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), which manages the IP address space allocations globally, cooperates with five Regional Internet Registries (RIRs) to allocate IP address blocks to Local Internet Registries (Internet service providers) and other entities.
IP versions
Two versions of the Internet Protocol (IP) are in use: IP Version 4 and IP Version 6. (See IP version history for details.) Each version defines an IP address differently. Because of its prevalence, the generic term IP address typically still refers to the addresses defined by IPv4.
An illustration of an IP address (version 4), in both dot-decimal notation and binary.
IP version 4 addresses
Main article: IPv4#Addressing
IPv4 uses 32-bit (4-byte) addresses, which limits the address space to 4,294,967,296 (232) possible unique addresses. IPv4 reserves some addresses for special purposes such as private networks (~18 million addresses) or multicast addresses (~270 million addresses).
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. In most representations each octet is converted individually.
IPv4 subnetting
In the early stages of development of the Internet Protocol,[1] network administrators interpreted an IP address in two parts, network number portion and host number portion. The highest order octet (most significant eight bits) in an address was designated the network number and the rest of the bits were called the rest field or host identifier and were used for host numbering within a network. This method soon proved inadequate as additional networks developed that were independent from the existing 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 network assignments. The first three bits of the most significant octet of an IP address was 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.Historical classful network architecture
Class First octet in binary Range of first octet Network ID Host ID Number of networks Number of addresses
A 0XXXXXXX 0 - 127 a b.c.d 27 = 128 224-2 = 16,777,214
B 10XXXXXX 128 - 191 a.b c.d 214 = 16,384 216-2 = 65,534
C 110XXXXX 192 - 223 a.b.c d 221 = 2,097,151 28-2 = 254
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) was 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.
IPv4 private addresses
Main article: Private network
Early network design, when global end-to-end connectivity was envisioned for communications with 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 public 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, when needed, such private networks typically connect to the Internet through network address translation (NAT).IANA-reserved private IPv4 network ranges
Start End No. of 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 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 - 192.168.0.255 (192.168.0.0/24).
IPv4 address depletion
Main article: IPv4 address exhaustion
The IP version 4 address space is rapidly nearing exhaustion of available, officially assignable address blocks.
IP version 6 addresses
Main article: IPv6 address
An illustration of an IP address (version 6), in hexadecimal and binary.
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) in 1995[3][4] The address size was increased from 32 to 128 bits or 16 octets, 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 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).
All modern desktop and enterprise server operating systems include native support for the IPv6 protocol, but it is not yet widely deployed in other devices, such as home networking routers, voice over Internet Protocol (VoIP) and multimedia equipment, and network peripherals.
Example of an IPv6 address:
2001:0db8:85a3:08d3:1319:8a2e:0370:7334
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.
IP subnetworks
Main article: Subnetwork
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.
Static vs 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.
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 or failure of static or stateful (DHCP) address configurations, an operating system may assign an IP address to a network interface using state-less autoconfiguration methods, such as Zeroconf.
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 an informal term used by cable and DSL Internet access subscribers to describe a dynamically assigned IP address that does not change often. The addresses are usually assigned with the DHCP protocol. Since the modems are usually powered-on for extended periods of time, the address leases are usually set to long periods and simply renewed upon expiration. If a modem is turned off and powered up again before the next expiration of the address lease, it will most likely receive the same IP address.
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.
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)
Modifications to IP addressing
IP blocking and firewalls
Main articles: IP blocking and Firewall (computer)
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
Main article: Network 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][clarification needed].
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.
Tools
In Windows the IP address can be determined by using the command-line tool ipconfig. In Unix the command-line ifconfig performs this function. ifconfig is available on Linux as well, though iproute2's "ip" command is sometimes more appropriate.
The IP address corresponding to a domain name can be determined by using nslookup example.net or dig example.net.
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