IP address

An Internet Protocol address (IP address) is a numerical label assigned to each device connected to a computer network that uses the Internet Protocol for communication.[1][2] An IP address serves two principal functions: host or network interface identification and location addressing.

Internet Protocol version 4 (IPv4) defines an IP address as a 32-bit number.[2] However, because of the growth of the Internet and the depletion of available IPv4 addresses, a new version of IP (IPv6), using 128 bits for the IP address, was developed in 1995,[3] and standardized in December 1998.[4] In July 2017, a final definition of the protocol was published.[5] IPv6 deployment has been ongoing since the mid-2000s.

IP addresses are usually written and displayed in human-readable notations, such as in IPv4, and 2001:db8:0:1234:0:567:8:1 in IPv6. The size of the routing prefix of the address is designated in CIDR notation by suffixing the address with the number of significant bits, e.g.,, which is equivalent to the historically used subnet mask

The IP address space is managed globally by the Internet Assigned Numbers Authority (IANA), and by five regional Internet registries (RIRs) responsible in their designated territories for assignment to end users and local Internet registries, such as Internet service providers. IPv4 addresses have been distributed by IANA to the RIRs in blocks of approximately 16.8 million addresses each. Each ISP or private network administrator assigns an IP address to each device connected to its network. Such assignments may be on a static (fixed or permanent) or dynamic basis, depending on its software and practices.


An IP address serves two principal functions. It identifies the host, or more specifically its network interface, and it provides the location of the host in the network, and thus the capability of establishing a path to that host. Its role 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 header of each IP packet contains the IP address of the sending host, and that of the destination host.

IP versions

Two versions of the Internet Protocol are in common use in the Internet today. The original version of the Internet Protocol that was first deployed in 1983 in the ARPANET, the predecessor of the Internet, is Internet Protocol version 4 (IPv4).

The rapid exhaustion of IPv4 address space available for assignment to Internet service providers and end user organizations by the early 1990s, prompted the Internet Engineering Task Force (IETF) to explore new technologies to expand the addressing capability in the Internet. The result was a redesign of the Internet Protocol which became eventually known as Internet Protocol Version 6 (IPv6) in 1995.[3][4][5] IPv6 technology was in various testing stages until the mid-2000s, when commercial production deployment commenced.

Today, these two versions of the Internet Protocol are in simultaneous use. Among other technical changes, each version defines the format of addresses differently. Because of the historical prevalence of IPv4, the generic term IP address typically still refers to the addresses defined by IPv4. The gap in version sequence between IPv4 and IPv6 resulted from the assignment of version 5 to the experimental Internet Stream Protocol in 1979, which however was never referred to as IPv5.


IP networks may be divided into subnetworks in both IPv4 and IPv6. For this purpose, an IP address is recognized as consisting of two parts: the network prefix in the high-order bits and the remaining bits called the rest field, host identifier, or interface identifier (IPv6), used for host numbering within a network.[1] The subnet mask or CIDR notation 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 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 and, respectively. The CIDR notation for the same IP address and subnet is, because the first 24 bits of the IP address indicate the network and subnet.

IPv4 addresses

Ipv4 address
Decomposition of an IPv4 address from dot-decimal notation to its binary value.

An IPv4 address has a size of 32 bits, which limits the address space to 4294967296 (232) addresses. Of this number, some addresses are reserved for special purposes such as private networks (~18 million addresses) and multicast addressing (~270 million addresses).

IPv4 addresses are usually represented in dot-decimal notation, consisting of four decimal numbers, each ranging from 0 to 255, separated by dots, e.g., Each part represents a group of 8 bits (an octet) of the address. In some cases of technical writing, IPv4 addresses may be presented in various hexadecimal, octal, or binary representations.

Subnetting history

In the early stages of development of the Internet Protocol, the network number was always the highest order octet (most significant eight bits). Because this method allowed for only 256 networks, it soon proved inadequate as additional networks developed that were independent of the existing networks already designated by a network number. In 1981, the addressing specification was revised with the introduction of classful network architecture.[2]

Classful network design allowed for a larger number of individual network assignments and fine-grained subnetwork design. The first three bits of the most significant octet of an IP address were 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 Leading
Size of network
bit field
Size of rest
bit field
of networks
Number of addresses
per network
Start address End address
A 0 8 24 128 (27) 16777216 (224)
B 10 16 16 16384 (214) 65536 (216)
C 110 24 8 2097152 (221) 256 (28)

Classful network design served its purpose in the startup stage of the Internet, but it lacked scalability in the face of the rapid expansion of networking in the 1990s. The class system of the address space was replaced with Classless Inter-Domain Routing (CIDR) in 1993. CIDR is based on variable-length subnet masking (VLSM) to allow allocation and routing based 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.

Private addresses

Early network design, when global end-to-end connectivity was envisioned for communications with all Internet hosts, intended that IP addresses be globally unique. However, it was found that this was not always necessary as private networks developed and public address space needed to be conserved.

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. Today, such private networks are widely used and typically connect to the Internet with network address translation (NAT), when needed.

Three non-overlapping ranges of IPv4 addresses for private networks are reserved.[6] These addresses are not routed on the Internet and thus their use need not be coordinated with an IP address registry. 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 through (

Reserved private IPv4 network ranges[6]
Name CIDR block Address range Number of addresses Classful description
24-bit block – 16777216 Single Class A.
20-bit block – 1048576 Contiguous range of 16 Class B blocks.
16-bit block – 65536 Contiguous range of 256 Class C blocks.

IPv6 addresses

Ipv6 address
Decomposition of an IPv6 address from hexadecimal representation to its binary value.

In IPv6, the address size was increased from 32 bits in IPv4 to 128 bits, thus providing up to 2128 (approximately 3.403×1038) addresses. This is deemed sufficient for the foreseeable future.

The intent of the new design was not to provide just a sufficient quantity of addresses, but also redesign routing in the Internet by allowing more efficient aggregation of subnetwork routing prefixes. This resulted in slower growth of routing tables in routers. 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 ratios will be small on any IPv6 network segment. The new design also provides the opportunity to separate the addressing infrastructure of a network segment, i.e. the local administration of the segment's available space, from the addressing prefix used to route traffic to and from external networks. 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 manual 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 no need to have complex address conservation methods as used in 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 residential networking routers, voice over IP (VoIP) and multimedia equipment, and some networking hardware.

Private addresses

Just as IPv4 reserves addresses for private networks, blocks of addresses are set aside in IPv6. In IPv6, these are referred to as unique local addresses (ULAs). The routing prefix fc00::/7 is reserved for this block,[7] which is divided into two /8 blocks with different implied policies. The addresses include a 40-bit pseudorandom number that minimizes the risk of address collisions if sites merge or packets are misrouted.

Early practices used a different block for this purpose (fec0::), dubbed site-local addresses.[8] However, the definition of what constituted a site remained unclear and the poorly defined addressing policy created ambiguities for routing. This address type was abandoned and must not be used in new systems.[9]

Addresses starting with fe80::, called link-local addresses, are assigned to interfaces for communication on the attached link. The addresses are automatically generated by the operating system for each network interface. This provides instant and automatic communication between all IPv6 host on a link. This feature is used in the lower layers of IPv6 network administration, such as for the Neighbor Discovery Protocol.

Private and link-local address prefixes may not be routed on the public Internet.

IP address assignment

IP addresses are assigned to a host either dynamically as they join the network, or persistently by configuration of the host hardware or software. Persistent configuration is also known as using a static IP address. In contrast, when a computer's IP address is assigned each time it restarts, this is known as using a dynamic IP address.

Dynamic IP addresses are assigned by network using Dynamic Host Configuration Protocol (DHCP). DHCP is the most frequently used technology for assigning addresses. It avoids the administrative burden of assigning specific static addresses to each device on a network. It also allows devices to share the limited address space on a network if only some of them are online at a particular time. Typically, dynamic IP configuration is enabled by default in modern desktop operating systems.

The address assigned with DHCP is associated with a lease and usually has an expiration period. If the lease is not renewed by the host before expiry, the address may be assigned to another device. Some DHCP implementations attempt to reassign the same IP address to a host (based on its MAC address) each time it joins the network. A network administrator may configure DHCP by allocating specific IP addresses based on MAC address.

DHCP is not the only technology used to assign IP addresses dynamically. Bootstrap Protocol is a similar protocol and predecessor to DHCP. Dialup and some broadband networks use dynamic address features of the Point-to-Point Protocol.

Computers and equipment used for the network infrastructure, such as routers and mail servers, are typically configured with static addressing.

In the absence or failure of static or dynamic address configurations, an operating system may assign a link-local address to a host using stateless address autoconfiguration.

Sticky dynamic IP address

A sticky dynamic IP address is an informal term used by cable and DSL Internet access subscribers to describe a dynamically assigned IP address which seldom changes. The addresses are usually assigned with DHCP. Since the modems are usually powered on for extended periods of time, the address leases are usually set to long periods and simply renewed. If a modem is turned off and powered up again before the next expiration of the address lease, it often receives the same IP address.

Address autoconfiguration

Address block is defined for the special use in link-local addressing for IPv4 networks.[10] In IPv6, every interface, whether using static or dynamic address assignments, also receives a link-local address automatically in the block fe80::/10.[10] These addresses are only valid on the link, such as a local network segment or point-to-point connection, to which a host is connected. 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 developed a protocol called Automatic Private IP Addressing (APIPA), whose first public implementation appeared in Windows 98.[11] APIPA has been deployed on millions of machines and became a de facto standard in the industry. Many years later, in May 2005, the IETF defined a formal standard for it.[12]

Addressing conflicts

An IP address conflict occurs when two devices on the same local physical or wireless network claim to have the same IP address. A second assignment of an address generally stops the IP functionality of one or both of the devices. Many modern operating systems notify the administrator of IP address conflicts.[13][14] When IP addresses are assigned by multiple people and systems with differing methods, any of them may be at fault.[15][16][17][18][19] If one of the devices involved in the conflict is the default gateway access beyond the LAN for all devices on the LAN may be impaired.


IP addresses are classified into several classes of operational characteristics: unicast, multicast, anycast and broadcast addressing.

Unicast addressing

The most common concept of an IP address is in unicast addressing, available in both IPv4 and IPv6. It normally refers to a single sender or a single receiver, and can be used for both sending and receiving. Usually, a unicast address is associated with a single device or host, but a device or host may have more than one unicast address. Some individual PCs have several distinct unicast addresses, each for its own distinct purpose. Sending the same data to multiple unicast addresses requires the sender to send all the data many times over, once for each recipient.

Broadcast addressing

Broadcasting is an addressing technique available in IPv4 to send data to all possible destinations on a network in one transmission operation, while all receivers capture the network packet (all-hosts broadcast). The address is used for network broadcast. In addition, a directed (limited) broadcast uses the all-ones host address with the network prefix. For example, the destination address used for directed broadcast to devices on the network is

IPv6 does not implement broadcast addressing, and replaces it with multicast to the specially defined all-nodes multicast address.

Multicast addressing

A multicast address is associated with a group of interested receivers. In IPv4, addresses through (the former Class D addresses) are designated as multicast addresses.[20] IPv6 uses the address block with the prefix ff00::/8 for multicast applications. In either case, the sender sends a single datagram from its unicast address to the multicast group address and the intermediary routers take care of making copies and sending them to all receivers that have joined the corresponding multicast group.

Anycast addressing

Like broadcast and multicast, anycast is a one-to-many routing topology. However, the data stream is not transmitted to all receivers, just the one which the router decides is logically closest in the network. Anycast address is an inherent feature of only IPv6. In IPv4, anycast addressing implementations typically operate using the shortest-path metric of BGP routing and do not take into account congestion or other attributes of the path. Anycast methods are useful for global load balancing and are commonly used in distributed DNS systems.


A host may use geolocation software to deduce the geolocation of its communicating peer.[21][22]

Public address

A public IP address, in common parlance, is a globally routable unicast IP address, meaning that the address is not an address reserved for use in private networks, such as those reserved by RFC 1918, or the various IPv6 address formats of local scope or site-local scope, for example for link-local addressing. Public IP addresses may be used for communication between hosts on the global Internet.


For security and privacy considerations, network administrators often desire to restrict public Internet traffic within their private networks. The source and destination IP addresses contained in the headers of each IP packet are a convenient means to discriminate traffic by IP address blocking or by selectively tailoring responses to external requests to internal servers. This is achieved with firewall software running on the network's gateway router. A database of IP addresses of permissible traffic may be maintained in blacklists or whitelists.

Address translation

Multiple client devices can appear to share an IP address, 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 the client, in which case the real originating IP address might be masked from the server receiving a request. A common practice is to have a NAT mask a large number of devices in a private network. Only the "outside" interface(s) of the NAT needs to have an Internet-routable address.[23]

Commonly, the NAT device maps TCP or UDP port numbers on the side of the larger, public network to individual private addresses on the masqueraded network.

In residential networks, NAT functions are usually implemented in a residential gateway. In this scenario, the computers connected to the router have private IP addresses and the router has a public address on its external interface to communicate on the Internet. The internal computers appear to share one public IP address.

Diagnostic tools

Computer operating systems provide various diagnostic tools to examine network interfaces and address configuration. Microsoft Windows provides the command-line interface tools ipconfig and netsh and users of Unix-like systems may use ifconfig, netstat, route, lanstat, fstat, and iproute2 utilities to accomplish the task.

See also


  1. ^ a b RFC 760, DOD Standard Internet Protocol, DARPA, Information Sciences Institute (January 1980).
  2. ^ a b c d J. Postel, ed. (September 1981). Internet Protocol, DARPA Internet Program Protocol Specification. IETF. doi:10.17487/RFC0791. RFC 791. Updated by RFC 1349, 2474, 6864.
  3. ^ a b S. Deering; R. Hinden (December 1995). Internet Protocol, Version 6 (IPv6) Specification. Network Working Group. doi:10.17487/RFC1883. RFC 1883.
  4. ^ a b S. Deering; R. Hinden (December 1998). Internet Protocol, Version 6 (IPv6) Specification. Network Working Group. doi:10.17487/RFC2460. RFC 2460.
  5. ^ a b S. Deering; R. Hinden (July 2017). Internet Protocol, Version 6 (IPv6) Specification. IETF. doi:10.17487/RFC8200. RFC 8200.
  6. ^ a b Y. Rekhter; B. Moskowitz; D. Karrenberg; G. J. de Groot; E. Lear (February 1996). Address Allocation for Private Internets. Network Working Group. doi:10.17487/RFC1918. BCP 5. RFC 1918. Updated by RFC 6761.
  7. ^ R. Hinden; B. Haberman (October 2005). Unique Local IPv6 Unicast Addresses. Network Working Group. doi:10.17487/RFC4193. RFC 4193.
  8. ^ R. Hinden; S. Deering (April 2003). Internet Protocol Version 6 (IPv6) Addressing Architecture. Network Working Group. doi:10.17487/RFC3513. RFC 3513. Obsoleted by RFC 4291.
  9. ^ C. Huitema; B. Carpenter (September 2004). Deprecating Site Local Addresses. Network Working Group. doi:10.17487/RFC3879. RFC 3879.
  10. ^ a b M. Cotton; L. Vegoda; R. Bonica; B. Haberman (April 2013). Special-Purpose IP Address Registries. Internet Engineering Task Force. doi:10.17487/RFC6890. BCP 153. RFC 6890. Updated by RFC 8190.
  11. ^ "DHCP and Automatic Private IP Addressing". docs.microsoft.com. Retrieved 20 May 2019.
  12. ^ S. Cheshire; B. Aboba; E. Guttman (May 2005). Dynamic Configuration of IPv4 Link-Local Addresses. Network Working Group. doi:10.17487/RFC3927. RFC 3927.
  13. ^ "Event ID 4198 — TCP/IP Network Interface Configuration". Microsoft. 7 January 2009. Archived from the original on 24 December 2013. Retrieved 2 June 2013. "Updated: January 7, 2009"
  14. ^ "Event ID 4199 — TCP/IP Network Interface Configuration". Microsoft. 7 January 2009. Archived from the original on 22 December 2013. Retrieved 2 June 2013. "Updated: 7 January 2009"
  15. ^ Mitchell, Bradley. "IP Address Conflicts – What Is an IP Address Conflict?". About.com. Archived from the original on 13 April 2014. Retrieved 23 November 2013.
  16. ^ Kishore, Aseem (4 August 2009). "How to Fix an IP Address Conflict". Online Tech Tips Online-tech-tips.com. Archived from the original on 25 August 2013. Retrieved 23 November 2013.
  17. ^ "Get help with "There is an IP address conflict" message". Microsoft. 22 November 2013. Archived from the original on 26 September 2013. Retrieved 23 November 2013.
  18. ^ "Fix duplicate IP address conflicts on a DHCP network". Microsoft. Archived from the original on 28 December 2014. Retrieved 23 November 2013. Article ID: 133490 – Last Review: 15 October 2013 – Revision: 5.0
  19. ^ Moran, Joseph (1 September 2010). "Understanding And Resolving IP Address Conflicts - Webopedia.com". Webopedia.com. Archived from the original on 2 October 2013. Retrieved 23 November 2013.
  20. ^ M. Cotton; L. Vegoda; D. Meyer (March 2010). IANA Guidelines for IPv4 Multicast Address Assignments. IETF. doi:10.17487/RFC5771. ISSN 2070-1721. BCP 51. RFC 5771.
  21. ^ "IP Information". 11 April 2013. Archived from the original on 4 July 2013. Retrieved 11 April 2013.
  22. ^ "NetAcuity Edge Offers Hyper-local IP targeting". 28 July 2009. Archived from the original on 5 February 2012. Retrieved 10 December 2011.
  23. ^ Comer, Douglas (2000). Internetworking with TCP/IP:Principles, Protocols, and Architectures – 4th ed. Upper Saddle River, NJ: Prentice Hall. p. 394. ISBN 978-0-13-018380-4. Archived from the original on 13 April 2010.

External links

Address Resolution Protocol

The Address Resolution Protocol (ARP) is a communication protocol used for discovering the link layer address, such as a MAC address, associated with a given internet layer address, typically an IPv4 address. This mapping is a critical function in the Internet protocol suite. ARP was defined in 1982 by RFC 826, which is Internet Standard STD 37.

ARP has been implemented with many combinations of network and data link layer technologies, such as IPv4, Chaosnet, DECnet and Xerox PARC Universal Packet (PUP) using IEEE 802 standards, FDDI, X.25, Frame Relay and Asynchronous Transfer Mode (ATM). IPv4 over IEEE 802.3 and IEEE 802.11 is the most common usage.

In Internet Protocol Version 6 (IPv6) networks, the functionality of ARP is provided by the Neighbor Discovery Protocol (NDP).

Broadcast address

A broadcast address is a network address at which all devices connected to a multiple-access communications network are enabled to receive datagrams. A message sent to a broadcast address may be received by all network-attached hosts.

In contrast, a multicast address is used to address a specific group of devices and a unicast address is used to address a single device.

A broadcast address may be an IP address at the network layer. In Ethernet networks, it can be a MAC address.

CNAME record

A Canonical Name record (abbreviated as CNAME record) is a type of resource record in the Domain Name System (DNS) which maps one domain name (an alias) to another (the Canonical Name.)This can prove convenient when running multiple services (like an FTP server and a web server, each running on different ports) from a single IP address. One can, for example, point ftp.example.com and www.example.com to the DNS entry for example.com, which in turn has an A record which points to the IP address. Then, if the IP address ever changes, one only has to record the change in one place within the network: in the DNS A record for example.com.

CNAME records must always point to another domain name, never directly to an IP address.

Classful network

A classful network is a network addressing architecture used in the Internet from 1981 until the introduction of Classless Inter-Domain Routing in 1993. The method divides the IP address space for Internet Protocol version 4 (IPv4) into five address classes based on the leading four address bits. Classes A, B, and C provide unicast addresses for networks of three different network sizes. Class D is for multicast networking and the class E address range is reserved for future or experimental purposes.

Since its discontinuation, remnants of classful network concepts have remained in practice only in limited scope in the default configuration parameters of some network software and hardware components, most notably in the default configuration of subnet masks.

Classless Inter-Domain Routing

Classless Inter-Domain Routing (CIDR ) is a method for allocating IP addresses and IP routing. The Internet Engineering Task Force introduced CIDR in 1993 to replace the previous addressing architecture of classful network design in the Internet. Its goal was to slow the growth of routing tables on routers across the Internet, and to help slow the rapid exhaustion of IPv4 addresses.IP addresses are described as consisting of two groups of bits in the address: the most significant bits are the network prefix, which identifies a whole network or subnet, and the least significant set forms the host identifier, which specifies a particular interface of a host on that network. This division is used as the basis of traffic routing between IP networks and for address allocation policies.

Whereas classful network design for IPv4 sized the network prefix as one or more 8-bit groups, resulting in the blocks of Class A, B, or C addresses, Classless Inter-Domain Routing allocates address space to Internet service providers and end users on any address bit boundary. In IPv6, however, the interface identifier has a fixed size of 64 bits by convention, and smaller subnets are never allocated to end users.

CIDR encompasses several concepts. It is based on the variable-length subnet masking (VLSM) technique, which allows the specification of arbitrary-length prefixes. CIDR introduced a new method of representation for IP addresses, now commonly known as CIDR notation, in which an address or routing prefix is written with a suffix indicating the number of bits of the prefix, such as for IPv4, and 2001:db8::/32 for IPv6. CIDR introduced an administrative process of allocating address blocks to organizations based on their actual and short-term projected needs. The aggregation of multiple contiguous prefixes resulted in supernets in the larger Internet, which whenever possible are advertised as aggregates, thus reducing the number of entries in the global routing table.

Domain name

A domain name is a label that identifies a network domain: a distinct group of computers under a central administration or authority.

Within the Internet, domain names are formed by the rules and procedures of the Domain Name System (DNS). Any name registered in the DNS is a domain name. Domain names are used in various networking contexts and for application-specific naming and addressing purposes. In general, a domain name represents an Internet Protocol (IP) resource, such as a personal computer used to access the Internet, a server computer hosting a web site, or the web site itself or any other service communicated via the Internet. In 2017, 330.6 million domain names had been registered.Domain names are organized in subordinate levels (subdomains) of the DNS root domain, which is nameless. The first-level set of domain names are the top-level domains (TLDs), including the generic top-level domains (gTLDs), such as the prominent domains com, info, net, edu, and org, and the country code top-level domains (ccTLDs). Below these top-level domains in the DNS hierarchy are the second-level and third-level domain names that are typically open for reservation by end-users who wish to connect local area networks to the Internet, create other publicly accessible Internet resources or run web sites.

The registration of these domain names is usually administered by domain name registrars who sell their services to the public.

A fully qualified domain name (FQDN) is a domain name that is completely specified with all labels in the hierarchy of the DNS, having no parts omitted. Labels in the Domain Name System are case-insensitive, and may therefore be written in any desired capitalization method, but most commonly domain names are written in lowercase in technical contexts.

Dynamic Host Configuration Protocol

The Dynamic Host Configuration Protocol (DHCP) is a network management protocol used on UDP/IP networks whereby a DHCP server dynamically assigns an IP address and other network configuration parameters to each device on a network so they can communicate with other IP networks. A DHCP server enables computers to request IP addresses and networking parameters automatically from the Internet service provider (ISP), reducing the need for a network administrator or a user to manually assign IP addresses to all network devices. In the absence of a DHCP server, a computer or other device on the network needs to be manually assigned an IP address, or to assign itself an APIPA address, which will not enable it to communicate outside its local subnet.

DHCP can be implemented on networks ranging in size from home networks to large campus networks and regional Internet service provider networks. A router or a residential gateway can be enabled to act as a DHCP server. Most residential network routers receive a globally unique IP address within the ISP network. Within a local network, a DHCP server assigns a local IP address to each device connected to the network.

Geolocation software

In computing, geolocation software is software that is capable of deducing the geolocation of a device connected to the Internet. The identification of a device's IP address can be used to determine the country, city, or post/ZIP code, determining an object's geographical location. Other methods include examination of a MAC address, image metadata, or credit card information.

IP address blocking

IP address blocking is a configuration of a network service so that requests from hosts with certain IP addresses are rejected.

Unix-like operating systems commonly implement IP address blocking using a TCP wrapper, configured by host access control files /etc/hosts.deny and /etc/hosts.allow.

IP address blocking is commonly used to protect against brute force attacks. Both companies and schools offering remote user access use Linux programs such as DenyHosts or Fail2ban for protection from unauthorized access while allowing permitted remote access. This is also useful for allowing remote access to computers. It is also used for Internet censorship.

On a website, an IP address block can prevent a disruptive address from access, though a warning and/or account block may be used first. Dynamic allocation of IP addresses by ISPs can complicate incoming IP address blocking, rendering it difficult to block a specific user without blocking many IP addresses (blocks of IP address ranges), thereby creating collateral damage.

IP address blocking can be used to restrict access to or from a particular geographic area—for example, the syndication of content to a specific region. To achieve this, IP addresses are mapped to the countries they have been assigned to. This has been used for example to target Nigerian IP addresses due to the perception that all business originating from the country is fraudulent, thus making it extremely difficult for legitimate businesses based in the country to interact with their counterparts in the rest of the world. To make purchases abroad, Nigerians must rely on proxy servers to disguise the true origin of an Internet request.

IP address spoofing

In computer networking, IP address spoofing or IP spoofing is the creation of Internet Protocol (IP) packets with a false source IP address, for the purpose of impersonating another computing system.

IPv4 address exhaustion

IPv4 address exhaustion is the depletion of the pool of unallocated IPv4 addresses. Because there are fewer than 4.3 billion addresses available, depletion has been anticipated since the late 1980s, when the Internet started to experience dramatic growth. This depletion is one of the reasons for the development and deployment of its successor protocol, IPv6. Currently IPv4 and IPv6 coexist in the Internet.

The IP address space is managed globally by the Internet Assigned Numbers Authority (IANA), and by five regional Internet registries (RIR) responsible in their designated territories for assignment to end users and local Internet registries, such as Internet service providers. The main market forces that accelerated IPv4 address depletion included the rapidly growing number of Internet users, always-on devices, and mobile devices.

The Internet Engineering Task Force (IETF) created the Routing and Addressing Group (ROAD) in November 1991 to respond to the scalability problem caused by the classful network allocation system in place at the time. The anticipated shortage has been the driving factor in creating and adopting several new technologies, including network address translation (NAT), Classless Inter-Domain Routing (CIDR) in 1993, and IPv6 in 1998. IPv6, the successor technology to IPv4 which was designed to address this problem, supports approximately 3.4×1038 network addresses. Although as of 2008 the predicted depletion was already approaching its final stages, most providers of Internet services and software vendors were just beginning IPv6 deployment at that time.The top-level exhaustion occurred on 31 January 2011. Four of the five RIRs have exhausted allocation of all the blocks they have not reserved for IPv6 transition; this occurred on 15 April 2011 for the Asia-Pacific (APNIC), on 14 September 2012 for Europe, Middle East and Central Asia (RIPE NCC), on 10 June 2014 for Latin America and the Caribbean (LACNIC), and on 24 September 2015 for North America (ARIN). Individual ISPs still had unassigned pools of IP addresses, and could recycle addresses no longer needed by their subscribers. Each exhausted its pool of available addresses at different times.

Internet Assigned Numbers Authority

The Internet Assigned Numbers Authority (IANA) is a function of ICANN, a nonprofit private American corporation that oversees global IP address allocation, autonomous system number allocation, root zone management in the Domain Name System (DNS), media types, and other Internet Protocol-related symbols and Internet numbers.Before ICANN was established primarily for this purpose in 1998, IANA was administered principally by Jon Postel at the Information Sciences Institute (ISI) of the University of Southern California (USC) situated at Marina Del Rey (Los Angeles), under a contract USC/ISI had with the United States Department of Defense, until ICANN was created to assume the responsibility under a United States Department of Commerce contract. Following ICANN's transition to a global multistakeholder governance model, the IANA functions were transferred to Public Technical Identifiers, an affiliate of ICANN.In addition, five regional Internet registries delegate number resources to their customers, local Internet registries, Internet service providers, and end-user organizations. A local Internet registry is an organization that assigns parts of its allocation from a regional Internet registry to other customers. Most local Internet registries are also Internet service providers.


In computer networking, localhost is a hostname that means this computer. It is used to access the network services that are running on the host via the loopback network interface. Using the loopback interface bypasses any local network interface hardware.

Network address translation

Network address translation (NAT) is a method of remapping one IP address space into another by modifying network address information in the IP header of packets while they are in transit across a traffic routing device. The technique was originally used as a shortcut to avoid the need to readdress every host when a network was moved. It has become a popular and essential tool in conserving global address space in the face of IPv4 address exhaustion. One Internet-routable IP address of a NAT gateway can be used for an entire private network.

IP masquerading is a technique that hides an entire IP address space, usually consisting of private IP addresses, behind a single IP address in another, usually public address space. The hidden addresses are changed into a single (public) IP address as the source address of the outgoing IP packets so they appear as originating not from the hidden host but from the routing device itself. Because of the popularity of this technique to conserve IPv4 address space, the term NAT has become virtually synonymous with IP masquerading.

As network address translation modifies the IP address information in packets. NAT implementations may vary in their specific behavior in various addressing cases and their effect on network traffic. The specifics of NAT behavior are not commonly documented by vendors of equipment containing NAT implementations.

Private network

In the Internet addressing architecture, a private network is a network that uses private IP address space. Both, the IPv4 and the IPv6 specifications define private addressing ranges. These addresses are commonly used for local area networks (LANs) in residential, office, and enterprise environments. Private IP address spaces were originally defined in an effort to delay IPv4 address exhaustion.

Private network addresses are not allocated to any specific organization and anyone may use these addresses without approval from a regional Internet registry. However, IP packets addressed from them cannot be routed through the public Internet.

Proxy server

In computer networks, a proxy server is a server (a computer system or an application) that acts as an intermediary for requests from clients seeking resources from other servers. A client connects to the proxy server, requesting some service, such as a file, connection, web page, or other resource available from a different server and the proxy server evaluates the request as a way to simplify and control its complexity. Proxies were invented to add structure and encapsulation to distributed systems.

Reverse DNS lookup

In computer networks, a reverse DNS lookup or reverse DNS resolution (rDNS) is the querying technique of the Domain Name System (DNS) to determine the domain name associated with an IP address – the reverse of the usual "forward" DNS lookup of an IP address from a domain name. The process of reverse resolving of an IP address uses PTR records. rDNS involves searching domain name registry and registrar tables. This may be used to try to identify the originator’s domain name to track, for example, a spammer sending spam emails or the domain name of a computer trying to break into a firewall or someone trying to hack the system. It may also be used to determine the name of the internet service provider assigned to a particular IP address. The reverse DNS database of the Internet is rooted in the .arpa top-level domain.

Although the informational RFC 1912 (Section 2.1) recommends that "every Internet-reachable host should have a name" and that "for every IP address, there should be a matching PTR record," it is not an Internet Standard requirement, and not all IP addresses have a reverse entry.


WHOIS (pronounced as the phrase "who is") is a query and response protocol that is widely used for querying databases that store the registered users or assignees of an Internet resource, such as a domain name, an IP address block or an autonomous system, but is also used for a wider range of other information. The protocol stores and delivers database content in a human-readable format. The current iteration of the WHOIS protocol was drafted by the Internet Society, and is documented in RFC 3912.


The X-Forwarded-For (XFF) HTTP header field is a common method for identifying the originating IP address of a client connecting to a web server through an HTTP proxy or load balancer.

The XFF HTTP request header was introduced by the Squid caching proxy server's developers. An RFC was proposed at the Internet Engineering Task Force (IETF).X-Forwarded-For is also an email-header indicating that an email-message was forwarded from one or more other accounts (probably automatically).In this context, the caching servers are most often those of large ISPs who either encourage or force their users to use proxy servers for access to the World Wide Web, something which is often done to reduce external bandwidth through caching. In some cases, these proxy servers are transparent proxies, and the user may be unaware that they are using them.

Without the use of XFF or another similar technique, any connection through the proxy would reveal only the originating IP address of the proxy server, effectively turning the proxy server into an anonymizing service, thus making the detection and prevention of abusive accesses significantly harder than if the originating IP address were available. The usefulness of XFF depends on the proxy server truthfully reporting the original host's IP address; for this reason, effective use of XFF requires knowledge of which proxies are trustworthy, for instance by looking them up in a whitelist of servers whose maintainers can be trusted.

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