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Encyclopedia > Domain name system
The five-layer TCP/IP model
5. Application layer

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The Domain Name System (DNS) associates various sorts of information with domain names; most importantly, it serves as the "phone book" for the Internet by translating human-readable computer hostnames, e.g. www.example.com, into the IP addresses, e.g. 208.77.188.166, that networking equipment needs to deliver information. It also stores other information such as the list of mail exchange servers that accept email for a given domain. In providing a worldwide keyword-based redirection service, the Domain Name System is an essential component of contemporary Internet use. In computing and telecommunications, the transport layer is the second highest layer in the four and five layer TCP/IP reference models, where it responds to service requests from the application layer and issues service requests to the Internet layer. ... The Transmission Control Protocol (TCP) is one of the core protocols of the Internet protocol suite. ... User Datagram Protocol (UDP) is one of the core protocols of the Internet protocol suite. ... The Datagram Congestion Control Protocol (DCCP) is a message-oriented transport layer protocol that is currently under development in the IETF. Applications that might make use of DCCP include those with timingconstraints on the delivery of data such that reliable in-order delivery, when combined with congestion control, is likely... In the field of computer networking, the IETF Signaling Transport (SIGTRAN) working group defined the Stream Control Transmission Protocol (SCTP) as a transport layer protocol in 2000. ... The Resource ReSerVation Protocol (RSVP), described in RFC 2205, is a Transport layer protocol designed to reserve resources across a network for an integrated services Internet. ... Network congestion avoidance is a process used in computer networks to avoid congestion. ... The network layer is third layer out of seven in OSI model and it is the third layer out of five in TCP/IP model. ... The Internet Protocol (IP) is a data-oriented protocol used for communicating data across a packet-switched internetwork. ... Internet Protocol version 4 is the fourth iteration of the Internet Protocol (IP) and it is the first version of the protocol to be widely deployed. ... Internet Protocol version 6 (IPv6) is a network layer for packet-switched internetworks. ... The Open Shortest Path First (OSPF) protocol is a hierarchical interior gateway protocol (IGP) for routing in Internet Protocol, using a link-state in the individual areas that make up the hierarchy. ... Is Is is Yeah Yeah Yeahs third EP, to be released on July 24, 2007. ... The Border Gateway Protocol (BGP) is the core routing protocol of the Internet. ... IPsec (IP security) is a suite of protocols for securing Internet Protocol (IP) communications by authenticating and/or encrypting each IP packet in a data stream. ... In computer networking, the Address Resolution Protocol (ARP) is the standard method for finding a hosts hardware address when only its network layer address is known. ... Reverse Address Resolution Protocol (RARP) is a network layer protocol used to obtain an IP address for a given hardware address (such as an Ethernet address). ... This article is chiefly about the Routing Information Protocol (RIP) for the Internet Protocol, but also discusses some other routing information protocols. ... The Internet Control Message Protocol (ICMP) is one of the core protocols of the Internet protocol suite. ... The ICMP for IPv6 (Internet Control Message Protocol Version 6) is an integral part of the IPv6 architecture and must be completely supported by all IPv6 implementations. ... The Internet Group Management Protocol (IGMP) is a communications protocol used to manage the membership of Internet Protocol multicast groups. ... This article does not cite any references or sources. ... IEEE 802. ... The IEEE 802. ... Wi-Fi (IPA: ) is the common name for a popular wireless technology used in home networks, mobile phones, video games and more. ... Official WiMax logo WiMAX, the Worldwide Interoperability for Microwave Access, is a telecommunications technology aimed at providing wireless data over long distances in a variety of ways, from point-to-point links to full mobile cellular type access. ... Asynchronous Transfer Mode (ATM) is a cell relay, packet switching network and data link layer protocol which encodes data traffic into small (53 bytes; 48 bytes of data and 5 bytes of header information) fixed-sized cells. ... Dynamic synchronous Transfer Mode , or DTM for short, is a network protocol. ... Token-Ring local area network (LAN) technology was developed and promoted by IBM in the early 1980s and standardised as IEEE 802. ... Ethernet is a large, diverse family of frame-based computer networking technologies that operate at many speeds for local area networks (LANs). ... In computer networking, fiber-distributed data interface (FDDI) is a standard for data transmission in a local area network that can extend in range up to 200 km (124 miles). ... In the context of computer networking, frame relay consists of an efficient data transmission technique used to send digital information quickly and cheaply in a relay of frames to one or many destinations from one or many end-points. ... General Packet Radio Service (GPRS) is a Mobile Data Service available to users of Global System for Mobile Communications (GSM) and IS-136 mobile phones. ... Evolution-Data Optimized or Evolution-Data only, abbreviated as EV-DO or EVDO and often EV, is one telecommunications standard for the wireless transmission of data through radio signals, typically for broadband Internet access. ... High-Speed Packet Access (HSPA) is a collection of mobile telephony protocols that extend and improve the performance of existing UMTS protocols. ... High-Level Data Link Control (HDLC) is a bit-oriented synchronous data link layer protocol developed by the International Organization for Standardization (ISO). ... In computing, the Point-to-Point Protocol, or PPP, is commonly used to establish a direct connection between two nodes. ... The Point-to-Point Tunneling Protocol (PPTP) is a method for implementing virtual private networks. ... In computer networking, the Layer 2 Tunneling Protocol (L2TP) is a tunneling protocol used to support virtual private networks (VPNs). ... ISDN redirects here. ... ARCNET (also CamelCased as ARCnet, an acronym from Attached Resource Computer NETwork) is a local area network (LAN) protocol, similar in purpose to Ethernet or Token Ring. ... Link Layer Topology Discovery (LLTD) is a licensed data link layer protocol for network topology discovery and quality of service diagnostics, developed by Microsoft as part of their Windows Rally set of technologies. ... This article does not cite any references or sources. ... The Ethernet physical layer is the physical layer component of the Ethernet standard. ... For other uses, see Modem (disambiguation). ... For other uses, see Power band. ... Synchronous optical networking, is a method for communicating digital information using lasers or light-emitting diodes (LEDs) over optical fiber. ... There are very few or no other articles that link to this one. ... Optical fibers An optical fiber (or fibre) is a glass or plastic fiber designed to guide light along its length. ... Coaxial Cable For the weapon, see coaxial weapon. ... 25 Pair Color Code Chart 10BASE-T UTP Cable Twisted pair cabling is a common form of wiring in which two conductors are wound around each other for the purposes of cancelling out electromagnetic interference known as crosstalk. ... The term domain name has multiple related meanings: A name that identifies a computer or computers on the Internet. ... Moscow phone book, 1930. ... A hostname (occasionally also, a sitename) is the unique name by which a network attached device (which could consist of a computer, file server, network storage device, fax machine, copier, cable modem, etc. ... This same page is served for example. ... An IP address (Internet Protocol address) is a unique address that certain electronic devices currently use in order to identify and communicate with each other on a computer network utilizing the Internet Protocol standard (IP)—in simpler terms, a computer address. ... A mail transfer agent or MTA (also called a mail server, or a mail exchange server in the context of the Domain Name System) is a computer program or software agent which transfers electronic mail messages from one computer to another. ... E-mail, or email, is short for electronic mail and is a method of composing, sending, and receiving messages over electronic communication systems. ... In computer science, a keyword is an identifier which indicates a specific command. ...

Contents

Uses

The most basic task of DNS is to translate hostnames to IP addresses. In very simple terms, it can be compared to a phone book. DNS also has other important uses. A hostname (occasionally also, a sitename) is the unique name by which a network attached device (which could consist of a computer, file server, network storage device, fax machine, copier, cable modem, etc. ... An IP address (Internet Protocol address) is a unique address that certain electronic devices currently use in order to identify and communicate with each other on a computer network utilizing the Internet Protocol standard (IP)—in simpler terms, a computer address. ...


Above all, DNS makes it possible to assign Internet names to organizations (or concerns they represent), independently of the physical routing hierarchy represented by the numerical IP address. Because of this, hyperlinks and Internet contact information can remain the same, whatever the current IP routing arrangements may be, and can take a human-readable form (such as "example.com"), which is easier to remember than the IP address 208.77.188.166. People take advantage of this when they recite meaningful URLs and e-mail addresses without caring how the machine will actually locate them. An IP address (Internet Protocol address) is a unique address that certain electronic devices currently use in order to identify and communicate with each other on a computer network utilizing the Internet Protocol standard (IP)—in simpler terms, a computer address. ... // Uniform Resource Locator (URL) formerly known as Universal Resource Locator, is a technical, Web-related term used in two distinct meanings: In popular usage and many technical documents, it is a synonym for Uniform Resource Identifier (URI); Strictly, the idea of a uniform syntax for global identifiers of network-retrievable... An e-mail address identifies a location to which e-mail messages can be delivered. ...


The Domain Name System distributes the responsibility for assigning domain names and mapping them to IP networks by allowing an authoritative server for each domain to keep track of its own changes, avoiding the need for a central registrar to be continually consulted and updated.


History

The practice of using a name as a more human-legible abstraction of a machine's numerical address on the network predates even TCP/IP, and goes all the way to the ARPAnet era. Back then however, a different system was used, as DNS was invented only in 1983, shortly after TCP/IP was deployed. With the older system, each computer on the network retrieved a file called HOSTS.TXT from a computer at SRI (now SRI International). The HOSTS.TXT file mapped numerical addresses to names. A hosts file still exists on most modern operating systems, either by default or through configuration, and allows users to specify an IP address (eg. 208.77.188.166) to use for a hostname (eg. www.example.net) without checking DNS. Systems based on a hosts file have inherent limitations, because of the obvious requirement that every time a given computer's address changed, every computer that seeks to communicate with it would need an update to its hosts file. The Internet protocol suite is the set of communications protocols that implement the protocol stack on which the Internet runs. ... ARPANET logical map, March 1977. ... SRI Internationals main campus on Ravenswood Avenue, Menlo Park, California SRI International is one of the worlds largest contract research institutions. ... The hosts file is a computer file used to store information on where to find a node on a computer network. ... An IP address (Internet Protocol address) is a unique address that certain electronic devices currently use in order to identify and communicate with each other on a computer network utilizing the Internet Protocol standard (IP)—in simpler terms, a computer address. ... A hostname (occasionally also, a sitename) is the unique name by which a network attached device (which could consist of a computer, file server, network storage device, fax machine, copier, cable modem, etc. ... example. ...


The growth of networking called for a more scalable system: one that recorded a change in a host's address in one place only. Other hosts would learn about the change dynamically through a notification system, thus completing a globally accessible network of all hosts' names and their associated IP Addresses.


At the request of Jon Postel, Paul Mockapetris invented the Domain Name system in 1983 and wrote the first implementation. The original specifications appear in RFC 882 and RFC 883. In November 1987, the publication of RFC 1034 and RFC 1035 updated the DNS specification[2] and made RFC 882 and RFC 883 obsolete. Several more-recent RFCs have proposed various extensions to the core DNS protocols. Jon Postel (Photo by Irene Fertik, USC News Service. ... Dr. Paul V. Mockapetris proposed a Domain Name System (DNS) architecture in 1983 in RFCs 882 and 883 while at the Information Sciences Institute (ISI) of the University of Southern California. ... 1987 is a common year starting on Thursday of the Gregorian calendar. ... In internetworking and computer network engineering, Request for Comments (RFC) documents are a series of memoranda encompassing new research, innovations, and methodologies applicable to Internet technologies. ...


In 1984, four Berkeley students — Douglas Terry, Mark Painter, David Riggle and Songnian Zhou — wrote the first UNIX implementation, which was maintained by Ralph Campbell thereafter. In 1985, Kevin Dunlap of DEC significantly re-wrote the DNS implementation and renamed it BIND (Berkeley Internet Name Domain, previously: Berkeley Internet Name Daemon). Mike Karels, Phil Almquist and Paul Vixie have maintained BIND since then. BIND was ported to the Windows NT platform in the early 1990s. Sather Tower (the Campanile) looking out over the San Francisco Bay and Mount Tamalpais. ... Filiation of Unix and Unix-like systems Unix (officially trademarked as UNIX®, sometimes also written as or ® with small caps) is a computer operating system originally developed in 1969 by a group of AT&T employees at Bell Labs including Ken Thompson, Dennis Ritchie and Douglas McIlroy. ... Digital Equipment Corporation was a pioneering American company in the computer industry. ... BIND (Berkeley Internet Name Domain, previously: Berkeley Internet Name Daemon) is the most commonly used DNS server on the Internet, especially on Unix-like systems, where it is a de facto standard. ... Paul Vixie is the author of several RFCs and well known UNIX system programs, among them SENDS, proxynet, rtty and Vixie cron. ... In computer science, porting is the process of adapting software so that an executable program can be created for a computing environment that is different from the one for which it was originally designed (e. ... Windows NT (New Technology) is a family of operating systems produced by Microsoft, the first version of which was released in July 1993. ...


Due to BIND's long history of security issues and exploits, several alternative nameserver and resolver programs have been written and distributed in recent years. This article is a comparison of DNS server software. ...


How DNS works in theory

Domain names, arranged in a tree, cut into zones, each served by a nameserver.
Domain names, arranged in a tree, cut into zones, each served by a nameserver.

Image File history File links Domain_name_space. ... Image File history File links Domain_name_space. ...

The domain name space

The domain name space consists of a tree of domain names. Each node or leaf in the tree has one or more resource records, which hold information associated with the domain name. The tree sub-divides into zones. A zone consists of a collection of connected nodes authoritatively served by an authoritative DNS nameserver. (Note that a single nameserver can host several zones.) In computer science, a tree is a widely-used computer data structure that emulates a tree structure with a set of linked nodes. ...


When a system administrator wants to let another administrator control a part of the domain name space within his zone of authority, he can delegate control to the other administrator. This splits a part of the old zone off into a new zone, which comes under the authority of the second administrator's nameservers. The old zone ceases to be authoritative for what goes under the authority of the new zone.


Parts of a domain name

A domain name usually consists of two or more parts (technically labels), separated by dots. For example example.com. The term domain name has multiple related meanings: A name that identifies a computer or computers on the Internet. ...

  • The rightmost label conveys the top-level domain (for example, the address www.example.com has the top-level domain com).
  • Each label to the left specifies a subdivision, or subdomain of the domain above it. Note;"subdomain" expresses relative dependence, not absolute dependence. For example: example.com comprises a subdomain of the com domain, and www.example.com comprises a subdomain of the domain example.com. In theory, this subdivision can go down to 127 levels deep. Each label can contain up to 63 characters. The whole domain name does not exceed a total length of 255 characters. In practice, some domain registries may have shorter limits.
  • A hostname refers to a domain name that has one or more associated IP addresses; ie: the www.example.com and example.com domains are both hostnames, however, the com domain is not.

“TLD” redirects here. ... In the Domain Name System (DNS) hierarchy, a subdomain is a domain that is part of a larger domain. ... A domain name registry, also called Network Information Centre (NIC), is part of the Domain Name System (DNS) of the Internet which converts domain names to IP addresses. ... A hostname (occasionally also, a sitename) is the unique name by which a network attached device (which could consist of a computer, file server, network storage device, fax machine, copier, cable modem, etc. ...

DNS servers

The Domain Name System consists of a hierarchical set of DNS servers. Each domain or subdomain has one or more authoritative DNS servers that publish information about that domain and the name servers of any domains "beneath" it. The hierarchy of authoritative DNS servers matches the hierarchy of domains. At the top of the hierarchy stand the root nameservers: the servers to query when looking up (resolving) a top-level domain name (TLD). The AMS-IX mirror of the K root-server. ... “TLD” redirects here. ...


DNS resolvers

A resolver looks up the resource record information associated with nodes. A resolver knows how to communicate with name servers by sending DNS queries and heeding DNS responses. Node(Latin nodus ‘knot’) is critical element of any computer network. ...


A DNS query may be either a recursive query or a non-recursive query:

  • A non-recursive query is one where the DNS server may provide a partial answer to the query (or give an error). DNS servers must support non-recursive queries.
  • A recursive query is one where the DNS server will fully answer the query (or give an error). DNS servers are not required to support recursive queries.

The resolver (or another DNS server acting recursively on behalf of the resolver) negotiates use of recursive service using bits in the query headers.


Resolving usually entails iterating through several name servers to find the needed information. However, some resolvers function simplistically and can only communicate with a single name server. These simple resolvers rely on a recursive query to a recursive name server to perform the work of finding information for them.


Address resolution mechanism

(This description deliberately uses the fictional .example TLD in accordance with the DNS guidelines themselves.)

In theory a full host name may have several name segments, (e.g ahost.ofasubnet.ofabiggernet.inadomain.example). In practice, in the experience of the majority of public users of Internet services, full host names will frequently consist of just three segments (ahost.inadomain.example, and most often www.inadomain.example).


For querying purposes, software interprets the name segment by segment, from right to left, using an iterative search procedure. At each step along the way, the program queries a corresponding DNS server to provide a pointer to the next server which it should consult.

A DNS recursor consults three nameservers to resolve the address www.wikipedia.org.
A DNS recursor consults three nameservers to resolve the address www.wikipedia.org.

As originally envisaged, the process was as simple as: Image File history File links An_example_of_theoretical_DNS_recursion. ... Image File history File links An_example_of_theoretical_DNS_recursion. ...

  1. the local system is pre-configured with the known addresses of the root servers in a file of root hints, which need to be updated periodically by the local administrator from a reliable source to be kept up to date with the changes which occur over time.
  2. query one of the root servers to find the server authoritative for the next level down (so in the case of our simple hostname, a root server would be asked for the address of a server with detailed knowledge of the example top level domain).
  3. querying this second server for the address of a DNS server with detailed knowledge of the second-level domain (inadomain.example in our example).
  4. repeating the previous step to progress down the name, until the final step which would, rather than generating the address of the next DNS server, return the final address sought.

The diagram illustrates this process for the real host www.wikipedia.org. The AMS-IX mirror of the K root-server. ...


The mechanism in this simple form has a difficulty: it places a huge operating burden on the root servers, with each and every search for an address starting by querying one of them. Being as critical as they are to the overall function of the system such heavy use would create an insurmountable bottleneck for trillions of queries placed every day. The section DNS in practice describes how this is addressed.


Circular dependencies and glue records

Name servers in delegations appear listed by name, rather than by IP address. This means that a resolving name server must issue another DNS request to find out the IP address of the server to which it has been referred. Since this can introduce a circular dependency if the nameserver referred to is under the domain that it is authoritative of, it is occasionally necessary for the nameserver providing the delegation to also provide the IP address of the next nameserver. This record is called a glue record. Circular dependencies is a situation which occurs in object oriented programming when two or more objects point towards each other in a circular fashion. ...


For example, assume that the sub-domain en.wikipedia.org contains further sub-domains (such as something.en.wikipedia.org) and that the authoritative nameserver for these lives at ns1.something.en.wikipedia.org. A computer trying to resolve something.en.wikipedia.org will thus first have to resolve ns1.something.en.wikipedia.org. Since ns1 is also under the something.en.wikipedia.org subdomain, resolving ns1.something.en.wikipedia.org requires resolving something.en.wikipedia.org which is exactly the circular dependency mentioned above. The dependency is broken by the glue record in the nameserver of en.wikipedia.org that provides the IP address of ns1.something.en.wikipedia.org directly to the requestor, enabling it to bootstrap the process by figuring out where ns1.something.en.wikipedia.org is located. In computing, Bootstrapping refers to a process where a simple system activates another more complicated system that serves the same purpose. ...


In practice

When an application (such as a web browser) tries to find the IP address of a domain name, it doesn't necessarily follow all of the steps outlined in the Theory section above. We will first look at the concept of caching, and then outline the operation of DNS in "the real world." An example of a Web browser (Mozilla Firefox) A web browser is a software application that enables a user to display and interact with text, images, videos, music and other information typically located on a Web page at a website on the World Wide Web or a local area network. ...


Caching and time to live

Because of the huge volume of requests generated by a system like DNS, the designers wished to provide a mechanism to reduce the load on individual DNS servers. To this end, the DNS resolution process allows for caching (i.e. the local recording and subsequent consultation of the results of a DNS query) for a given period of time after a successful answer. How long a resolver caches a DNS response (i.e. how long a DNS response remains valid) is determined by a value called the time to live (TTL). The TTL is set by the administrator of the DNS server handing out the response. The period of validity may vary from just seconds to days or even weeks. Time to live (sometimes abbreviated TTL) is a limit on the period of time or number of iterations or transmissions in computer and computer network technology that a unit of data (e. ...


Caching time

As a noteworthy consequence of this distributed and caching architecture, changes to DNS do not always take effect immediately and globally. This is best explained with an example: If an administrator has set a TTL of 6 hours for the host www.wikipedia.org, and then changes the IP address to which www.wikipedia.org resolves at 12:01pm, the administrator must consider that a person who cached a response with the old IP address at 12:00pm will not consult the DNS server again until 6:00pm. The period between 12:01pm and 6:00pm in this example is called caching time, which is best defined as a period of time that begins when you make a change to a DNS record and ends after the maximum amount of time specified by the TTL expires. This essentially leads to an important logistical consideration when making changes to DNS: not everyone is necessarily seeing the same thing you're seeing. RFC 1537 helps to convey basic rules for how to set the TTL. Time to live (sometimes abbreviated TTL) is a limit on the period of time or number of iterations or transmissions in computer and computer network technology that a unit of data (e. ... Time to live (sometimes abbreviated TTL) is a limit on the period of time or number of iterations or transmissions in computer and computer network technology that a unit of data (e. ...


Note that the term "propagation", although very widely used in this context, does not describe the effects of caching well. Specifically, it implies that [1] when you make a DNS change, it somehow spreads to all other DNS servers (instead, other DNS servers check in with yours as needed), and [2] that you do not have control over the amount of time the record is cached (you control the TTL values for all DNS records in your domain, except your NS records and any authoritative DNS servers that use your domain name).


Some resolvers may override TTL values, as the protocol supports caching for up to 68 years or no caching at all. Negative caching (the non-existence of records) is determined by name servers authoritative for a zone which MUST include the Start of Authority (SOA) record when reporting no data of the requested type exists. The MINIMUM field of the SOA record and the TTL of the SOA itself is used to establish the TTL for the negative answer. RFC 2308


Many people incorrectly refer to a mysterious 48 hour or 72 hour propagation time when you make a DNS change. When one changes the NS records for one's domain or the IP addresses for hostnames of authoritative DNS servers using one's domain (if any), there can be a lengthy period of time before all DNS servers use the new information. This is because those records are handled by the zone parent DNS servers (for example, the .com DNS servers if your domain is example.com), which typically cache those records for 48 hours. However, those DNS changes will be immediately available for any DNS servers that do not have them cached. And any DNS changes on your domain other than the NS records and authoritative DNS server names can be nearly instantaneous, if you choose for them to be (by lowering the TTL once or twice ahead of time, and waiting until the old TTL expires before making the change).


In the real world

DNS resolving from program to OS-resolver to ISP-resolver to greater system.
DNS resolving from program to OS-resolver to ISP-resolver to greater system.

Users generally do not communicate directly with a DNS resolver. Instead DNS-resolution takes place transparently in client-applications such as web-browsers, mail-clients, and other Internet applications. When an application makes a request which requires a DNS lookup, such programs send a resolution request to the local DNS resolver in the local operating system, which in turn handles the communications required. Image File history File links DNS_in_the_real_world. ... Image File history File links DNS_in_the_real_world. ... An example of a Web browser (Mozilla Firefox) A web browser is a software application that enables a user to display and interact with text, images, videos, music and other information typically located on a Web page at a website on the World Wide Web or a local area network. ... This article does not cite any references or sources. ... In Unix/Linux computer systems, resolver is a set of software routines used for making, sending and interpreting query and reply messages with Internet domain name servers. ...


The DNS resolver will almost invariably have a cache (see above) containing recent lookups. If the cache can provide the answer to the request, the resolver will return the value in the cache to the program that made the request. If the cache does not contain the answer, the resolver will send the request to one or more designated DNS servers. In the case of most home users, the Internet service provider to which the machine connects will usually supply this DNS server: such a user will either have configured that server's address manually or allowed DHCP to set it; however, where systems administrators have configured systems to use their own DNS servers, their DNS resolvers point to separately maintained nameservers of the organization. In any event, the name server thus queried will follow the process outlined above, until it either successfully finds a result or does not. It then returns its results to the DNS resolver; assuming it has found a result, the resolver duly caches that result for future use, and hands the result back to the software which initiated the request. “ISP” redirects here. ... DHCP redirects here. ...


Broken resolvers

An additional level of complexity emerges when resolvers violate the rules of the DNS protocol. A number of large ISPs have configured their DNS servers to violate rules (presumably to allow them to run on less-expensive hardware than a fully-compliant resolver), such as by disobeying TTLs, or by indicating that a domain name does not exist just because one of its name servers does not respond.[citation needed]


As a final level of complexity, some applications (such as web-browsers) also have their own DNS cache, in order to reduce the use of the DNS resolver library itself. This practice can add extra difficulty when debugging DNS issues, as it obscures the freshness of data, and/or what data comes from which cache. These caches typically use very short caching times — on the order of one minute. Internet Explorer offers a notable exception: recent versions cache DNS records for half an hour.[1] Windows Internet Explorer (formerly Microsoft Internet Explorer abbreviated MSIE), commonly abbreviated to IE, is a series of graphical web browsers developed by Microsoft and included as part of the Microsoft Windows line of operating systems starting in 1995. ... 2007 is a common year starting on Monday of the Gregorian calendar. ...


Other applications

The system outlined above provides a somewhat simplified scenario. The Domain Name System includes several other functions:

  • Hostnames and IP addresses do not necessarily match on a one-to-one basis. Many hostnames may correspond to a single IP address: combined with virtual hosting, this allows a single machine to serve many web sites. Alternatively a single hostname may correspond to many IP addresses: this can facilitate fault tolerance and load distribution, and also allows a site to move physical location seamlessly.
  • There are many uses of DNS besides translating names to IP addresses. For instance, Mail transfer agents use DNS to find out where to deliver e-mail for a particular address. The domain to mail exchanger mapping provided by MX records accommodates another layer of fault tolerance and load distribution on top of the name to IP address mapping.
  • Sender Policy Framework and DomainKeys instead of creating their own record types were designed to take advantage of another DNS record type, the TXT record.
  • To provide resilience in the event of computer failure, multiple DNS servers are usually provided for coverage of each domain, and at the top level, thirteen very powerful root servers exist, with additional "copies" of several of them distributed worldwide via Anycast.

It has been suggested that this article or section be merged into shared web hosting service. ... In computer science, Fault-tolerance is the property of a computer system to continue operation at an acceptable quality, despite the unexpected occurrence of hardware or software failures. ... A mail transfer agent or MTA (also called a mail transport agent, mail server, or a mail exchanger in the context of the Domain Name System) is a computer program or software agent that transfers electronic mail messages from one computer to another. ... Wikipedia does not yet have an article with this exact name. ... An MX record or Mail exchanger record is a type of resource record in the Domain Name System (DNS) specifying how Internet e-mail should be routed. ... In computing, Sender Policy Framework (SPF) is an extension to the Simple Mail Transfer Protocol (SMTP). ... DomainKeys is an e-mail authentication system designed to verify the DNS domain of an E-mail sender and the message integrity. ... The AMS-IX mirror of the K root-server. ... Routing Schemes anycast broadcast multicast unicast Anycast is a network addressing and routing scheme whereby data is routed to the nearest or best destination as viewed by the routing topology. ...

Protocol details

DNS primarily uses UDP on port 53 [2] to serve requests. Almost all DNS queries consist of a single UDP request from the client followed by a single UDP reply from the server. TCP comes into play only when the response data size exceeds 512 bytes, or for such tasks as zone transfer. Some operating systems such as HP-UX are known to have resolver implementations that use TCP for all queries, even when UDP would suffice. User Datagram Protocol (UDP) is one of the core protocols of the Internet protocol suite. ... It has been suggested that this article or section be merged into Computer port (software). ... The Transmission Control Protocol (TCP) is one of the core protocols of the Internet protocol suite. ... DNS zone transfer, also sometimes referred known by its (commonest) opcode mnemonic AXFR, is a type of DNS transaction. ... HP-UX (Hewlett Packard UniX) is Hewlett-Packards proprietary implementation of the Unix operating system, based on System V (initially System III). ...


Extensions to DNS

EDNS is an extension of the DNS protocol which allows the transport over UDP of DNS replies exceeding 512 bytes, and adds support for expanding the space of request and response codes. It is described in RFC 2671. This article or section is in need of attention from an expert on the subject. ...


Standards

A number of the aspects of the Domain name system are specified by an Internet standard. The following is a list of some of the RFCs that pertain to DNS. An Internet standard is a specification for an innovative internetworking technology or methodology, which the Internet Engineering Task Force (IETF) ratified as an open standard after the innovation underwent peer review. ... In internetworking and computer network engineering, Request for Comments (RFC) documents are a series of memoranda encompassing new research, innovations, and methodologies applicable to Internet technologies. ...

  • RFC 882 Concepts and Facilities (Deprecated by RFC 1034)
  • RFC 883 Domain Names: Implementation specification (Deprecated by RFC 1035)
  • RFC 920 Specified original TLDs: .arpa, .com, .edu, .org, .gov, .mil and two-character country codes
  • RFC 1032 Domain administrators guide
  • RFC 1033 Domain administrators operations guide
  • RFC 1034 Domain Names - Concepts and Facilities.
  • RFC 1035 Domain Names - Implementation and Specification
  • RFC 1101 DNS Encodings of Network Names and Other Types
  • RFC 1123 Requirements for Internet Hosts -- Application and Support
  • RFC 1183 New DNS RR Definitions
  • RFC 1706 DNS NSAP Resource Records
  • RFC 1876 Location Information in the DNS (LOC)
  • RFC 1886 DNS Extensions to support IP version 6
  • RFC 1912 Common DNS Operational and Configuration Errors
  • RFC 1995 Incremental Zone Transfer in DNS
  • RFC 1996 A Mechanism for Prompt Notification of Zone Changes (DNS NOTIFY)
  • RFC 2136 Dynamic Updates in the domain name system (DNS UPDATE)
  • RFC 2181 Clarifications to the DNS Specification
  • RFC 2182 Selection and Operation of Secondary DNS Servers
  • RFC 2308 Negative Caching of DNS Queries (DNS NCACHE)
  • RFC 2317 Classless IN-ADDR.ARPA delegation
  • RFC 2671 Extension Mechanisms for DNS (EDNS0)
  • RFC 2672 Non-Terminal DNS Name Redirection (DNAME record)
  • RFC 2782 A DNS RR for specifying the location of services (DNS SRV)
  • RFC 2845 Secret Key Transaction Authentication for DNS (TSIG)
  • RFC 2874 DNS Extensions to Support IPv6 Address Aggregation and Renumbering
  • RFC 3403 Dynamic Delegation Discovery System (DDDS) (NAPTR records)
  • RFC 3696 Application Techniques for Checking and Transformation of Names
  • RFC 4398 Storing Certificates in the Domain Name System
  • RFC 4408 Sender Policy Framework (SPF) (SPF records)

The LOC record (RFC1876) is a means for expressing location information in the Domain Name System. ... Internet Protocol version 6 (IPv6) is a network layer for packet-switched internetworks. ... An SRV record or Service record is a category of data in the Internet Domain Name System specifying information on available services. ... TSIG (Transaction SIGnature) is a computer networking protocol. ... NAPTR stands for Naming Authority Pointer and is a newer type of DNS record that supports regular expression based rewriting. ... In computing, Sender Policy Framework (SPF) is an extension to the Simple Mail Transfer Protocol (SMTP). ...

Types of DNS records

Main article: List of DNS record types

Important categories of data stored in DNS include the following: Image File history File links Mergefrom. ... The List of DNS record types provides a convenient index to information about the various kinds of DNS records. ... The List of DNS record types provides a convenient index to information about the various kinds of DNS records. ...

  1. An A record or address record maps a hostname to a 32-bit IPv4 address.
  2. An AAAA record or IPv6 address record maps a hostname to a 128-bit IPv6 address.
  3. A CNAME record or canonical name record is an alias of one name to another. The A record to which the alias points can be either local or remote - on a foreign name server. This is useful when running multiple services (such as an FTP and a webserver) from a single IP address. Each service can then have its own entry in DNS (like ftp.example.com. and www.example.com.)
  4. An MX record or mail exchange record maps a domain name to a list of mail exchange servers for that domain.
  5. A PTR record or pointer record maps an IPv4 address to the canonical name for that host. Setting up a PTR record for a hostname in the in-addr.arpa domain that corresponds to an IP address implements reverse DNS lookup for that address.
  6. An NS record or name server record maps a domain name to a list of DNS servers authoritative for that domain. Delegations depend on NS records.
  7. An SOA record or start of authority record specifies the DNS server providing authoritative information about an Internet domain, the email of the domain administrator, the domain serial number, and several timers relating to refreshing the zone.
  8. An SRV record is a generalized service location record.
  9. A TXT Record was originally intended to carry arbitrary human-readable text in a DNS record. Since the early 1990s, however, this record is more often used to carry machine-readable data such as specified by RFC 1464, opportunistic encryption, Sender Policy Framework and DomainKeys.
  10. An NAPTR record ("Naming Authority Pointer") is a newer type of DNS record that support regular expression based rewriting.
  11. An SPF record is a new, experimental record used as part of the Sender Policy Framework to identify forged addresses in the 'envelope sender' address of an email transaction (common in e-mail spam).

Other types of records are not widely used. For example, a LOC record gives the physical location of a host; a WKS record gives a bit list of well known service (such as HTTP or POP3) running on a given host[3]. Internet Protocol version 4 is the fourth iteration of the Internet Protocol (IP) and it is the first version of the protocol to be widely deployed. ... Internet Protocol version 6 (IPv6) is a network layer for packet-switched internetworks. ... Internet Protocol version 6 (IPv6) is a network layer for packet-switched internetworks. ... An MX record or Mail exchanger record is a type of resource record in the Domain Name System (DNS) specifying how Internet e-mail should be routed. ... A mail transfer agent or MTA (also called a mail server, or a mail exchange server in the context of the Domain Name System) is a computer program or software agent which transfers electronic mail messages from one computer to another. ... Internet Protocol version 4 is the fourth iteration of the Internet Protocol (IP) and it is the first version of the protocol to be widely deployed. ... A fully qualified domain name (or FQDN) is the human-readable name corresponding to the TCP/IP address of a network interface, as found on a computer, router or other networked device. ... Reverse DNS lookup (rDNS) is a process to determine the hostname or host associated with a given IP address or host address. ... An SRV record or Service record is a category of data in the Internet Domain Name System specifying information on available services. ... Opportunistic Encryption (OE) allows for encryption for secure communication without any pre-arrangement specific to the pair of systems involved. ... In computing, Sender Policy Framework (SPF) is an extension to the Simple Mail Transfer Protocol (SMTP). ... DomainKeys is an e-mail authentication system designed to verify the DNS domain of an E-mail sender and the message integrity. ... NAPTR stands for Naming Authority Pointer and is a newer type of DNS record that supports regular expression based rewriting. ... In computing, Sender Policy Framework (SPF) is an extension to the Simple Mail Transfer Protocol (SMTP). ... E-mail spam, also known as bulk e-mail or junk e-mail is a subset of spam that involves sending nearly identical messages to numerous recipients by e-mail. ... The LOC record (RFC1876) is a means for expressing location information in the Domain Name System. ...


When sent over the internet, all records use the common format specified in RFC 1035 shown below.

RR (Resource record) fields
Field Description Length (octets)
NAME Name of the node to which this record pertains. (variable)
TYPE Type of RR. For example, MX is type 15. 2
CLASS Class code. 2
TTL Signed time in seconds that RR stays valid. 4
RDLENGTH Length of RDATA field. 2
RDATA Additional RR-specific data. (variable)

For a complete list of DNS record types consult IANA DNS Parameters In computing, an octet is a grouping of eight bits. ... Time to live (sometimes abbreviated TTL) is a limit on the period of time or number of iterations or transmissions in computer and computer network technology that a unit of data (e. ...


Internationalized domain names

While domain names technically have no restrictions on the characters they use and can include non-ASCII characters, the same is not true for host names.[4] Host names are the names most people see and use for things like e-mail and web browsing. Host names are restricted to a small subset of the ASCII character set that includes the Roman alphabet in upper and lower case, the digits 0 through 9, the dot, and the hyphen. (See RFC 3696 section 2 for details.) This prevented the representation of names and words of many languages natively. ICANN has approved the Punycode-based IDNA system, which maps Unicode strings into the valid DNS character set, as a workaround to this issue. Some registries have adopted IDNA. Example of Arabic IDN Example of Chinese IDN Example of Persian IDN Example of Greek IDN Example of Hebrew IDN Example of Ukrainian IDN An internationalized domain name (IDN) is an Internet domain name that (potentially) contains non-ASCII characters. ... Image:ASCII fullsvg There are 95 printable ASCII characters, numbered 32 to 126. ... The Latin alphabet, also called the Roman alphabet, is the most widely used alphabetic writing system in the world today. ... This article is about the punctuation mark. ... ICANN headquarters ICANN (IPA /aɪkæn/) is the Internet Corporation for Assigned Names and Numbers. ... Punycode is a computer programming encoding syntax by which a Unicode string of characters can be translated into the more-limited character set permitted in network host names. ... Example of Arabic IDN Example of Chinese IDN Example of Persian IDN Example of Greek IDN Example of Hebrew IDN Example of Ukrainian IDN An internationalized domain name (IDN) is an Internet domain name that (potentially) contains non-ASCII characters. ... The Unicode Standard, Version 5. ... A domain name registry, also called Network Information Centre (NIC), is part of the Domain Name System (DNS) of the Internet which converts domain names to IP addresses. ...


Security issues

DNS was not originally designed with security in mind, and thus has a number of security issues.


One class of vulnerabilities is DNS cache poisoning, which tricks a DNS server into believing it has received authentic information when, in reality, it has not. DNS cache poisoning is a technique that tricks a DNS server into believing it has received authentic information when, in reality, it has not. ...


DNS responses are traditionally not cryptographically signed, leading to many attack possibilities; DNSSEC modifies DNS to add support for cryptographically signed responses. There are various extensions to support securing zone transfer information as well. The Domain Name System Security Extensions (DNSSEC) are a suite of IETF specifications for securing certain kinds of information provided by the Domain Name System (DNS) as used on Internet Protocol (IP) networks. ...


Even with encryption, a DNS server could become compromised by a virus (or for that matter a disgruntled employee) that would cause IP addresses of that server to be redirected to a malicious address with a long TTL. This could have far-reaching impact to potentially millions of Internet users if busy DNS servers cache the bad IP data. This would require manual purging of all affected DNS caches as required by the long TTL (up to 68 years).


Some domain names can spoof other, similar-looking domain names. For example, "paypal.com" and "paypa1.com" are different names, yet users may be unable to tell the difference when the user's typeface (font) does not clearly differentiate the letter l and the number 1. This problem is much more serious in systems that support internationalized domain names, since many characters that are different, from the point of view of ISO 10646, appear identical on typical computer screens. This vulnerability is often exploited in phishing. “Font” redirects here. ... For other uses, see L (disambiguation). ... This article is about the number one. ... Example of Arabic IDN Example of Chinese IDN Example of Persian IDN Example of Greek IDN Example of Hebrew IDN Example of Ukrainian IDN An internationalized domain name (IDN) is an Internet domain name that (potentially) contains non-ASCII characters. ... The Universal Character Set is a character encoding that is defined by the international standard ISO/IEC 10646. ... An example of a phishing email, disguised as an official email from a (fictional) bank. ...


Techniques such as Forward Confirmed reverse DNS can also be used to help validate DNS results. FCrDNS, or Forward Confirmed Reverse DNS, is when an IP address has both forward (name -> IP) and reverse (IP -> name) DNS entries that match each other. ...


Legal users of domains

Registrant

Most of the domain name registrars in the world receive an annual fee from a legal user in order for the legal user to utilize the domain name (i.e. a sort of leasing agreement exists, subject to the registry's terms and conditions). Depending on the various naming conventions of the registries, legal users are commonly known as "registrants" or as "domain holders". A domain name registrar is a company accredited, either by the Internet Corporation for Assigned Names and Numbers (ICANN), or by a national ccTLD authority or both, to register Internet domain names . ...


ICANN holds a complete list of domain registries in the world. One can obtain information about the legal user of a domain name by looking in the WHOIS database held by most domain registries. ICANN headquarters ICANN (IPA /aɪkæn/) is the Internet Corporation for Assigned Names and Numbers. ... WHOIS is a TCP-based query/response protocol which is widely used for querying a database in order to determine the owner of a domain name, an IP address, or an autonomous system number on the Internet. ...


For most of the more than 240 country code top-level domains (ccTLDs), the domain registries hold the authoritative WHOIS (Registrant, name servers, expiration dates, etc.). For instance, DENIC, Germany NIC, holds the authoritative WHOIS to a .DE domain name. Since about 2001, most gTLD registries (.ORG, .BIZ, .INFO) have adopted this so-called "thick" registry approach, i.e. keeping the authoritative WHOIS in the central registries instead of the registrars. A country code top-level domain (ccTLD) is a top-level domain used and reserved for a country or a dependent territory. ... DENIC Verwaltungs- und Betriebsgesellschaft eG is the manager of the . ... A generic top-level domain (gTLD) is a top-level domain used (at least in theory) by a particular class of organization. ... WHOIS is a TCP-based query/response protocol which is widely used for querying a database in order to determine the owner of a domain name, an IP address, or an autonomous system number on the Internet. ...


For .COM and .NET domain names, a "thin" registry is used: the domain registry (e.g. VeriSign) holds a basic WHOIS (registrar and name servers, etc.). One can find the detailed WHOIS (registrant, name servers, expiry dates, etc.) at the registrars. WHOIS is a TCP-based query/response protocol which is widely used for querying a database in order to determine the owner of a domain name, an IP address, or an autonomous system number on the Internet. ... A name server is a computer server that implements a name service protocol. ...


Some domain name registries, also called Network Information Centres (NIC), also function as registrars, and deal directly with end users. But most of the main ones, such as for .COM, .NET, .ORG, .INFO, etc., use a registry-registrar model. There are hundreds of Domain Name Registrars that actually perform the domain name registration with the end user (see lists at ICANN or VeriSign). By using this method of distribution, the registry only has to manage the relationship with the registrar, and the registrar maintains the relationship with the end users, or 'registrants' -- in some cases through additional layers of resellers.


Administrative contact

A registrant usually designates an administrative contact to manage the domain name. In practice, the administrative contact usually has the most immediate power over a domain. Management functions delegated to the administrative contacts may include (for example):

  • the obligation to conform to the requirements of the domain registry in order to retain the right to use a domain name
  • authorization to update the physical address, e-mail address and telephone number etc. in WHOIS

WHOIS is a TCP-based query/response protocol which is widely used for querying a database in order to determine the owner of a domain name, an IP address, or an autonomous system number on the Internet. ...

Technical contact

A technical contact manages the name servers of a domain name. The many functions of a technical contact include:

  • making sure the configurations of the domain name conforms to the requirements of the domain registry
  • updating the domain zone
  • providing the 24×7 functionality of the name servers (that leads to the accessibility of the domain name)

Billing contact

The party whom a domain name registrar invoices. A domain name registrar is a company accredited, either by the Internet Corporation for Assigned Names and Numbers (ICANN), or by a national ccTLD authority or both, to register Internet domain names . ...


Name servers

Namely the authoritative name servers that host the domain name zone of a domain name. A name server is a computer server that implements a name service protocol. ...


Politics

Critics commonly claim abuse of domains by monopolies or near-monopolies, such as VeriSign, Inc. Particularly noteworthy was the VeriSign Site Finder system which redirected all unregistered .com and .net domains to a VeriSign webpage. For example, at a public meeting with VeriSign to air technical concerns about SiteFinder [5], numerous people, active in the IETF and other technical bodies, explained how they were surprised by VeriSign's changing the fundamental behavior of a major component of Internet infrastructure, not having obtained the customary consensus. SiteFinder, at first, assumed every Internet query was for a website, and it monetized queries for incorrect domain names, taking the user to VeriSign's search site. Unfortunately, other applications, such as many implementations of email, treat a lack of response to a domain name query as an indication that the domain does not exist, and that the message can be treated as undeliverable. The original VeriSign implementation broke this assumption for mail, because it would always resolve an erroneous domain name to that of SiteFinder. While VeriSign later changed SiteFinder to have this behavior only in response to true Web queries, there was still widespread protest about VeriSign's action being more in its financial interest than in the interest of the Internet infrastructure component for which VeriSign was the steward. VeriSign, Inc. ... Site Finder was a wildcard DNS record for all . ... The Internet Engineering Task Force (IETF) develops and promotes Internet standards, cooperating closely with the W3C and ISO/IEC standard bodies; and dealing in particular with standards of the TCP/IP and Internet protocol suite. ...


Despite widespread criticism, VeriSign only reluctantly removed it after the Internet Corporation for Assigned Names and Numbers (ICANN) threatened to revoke its contract to administer the root name servers. ICANN published the extensive set of letters exchanged, committee reports, and ICANN decisions [6]. ICANN is the Internet Corporation for Assigned Names and Numbers. ...


There is also significant disquiet regarding the United States' political influence over ICANN. This was a significant issue in the attempt to create a .xxx top-level domain and sparked greater interest in alternative DNS roots that would be beyond the control of any single country.[citation needed] .xxx is a proposed top-level domain (TLD) intended as a voluntary option for sexually explicit sites on the Internet. ... “TLD” redirects here. ... In addition to the Internets main DNS root (currently consisting of 13 nominal root nameservers working in agreement with ICANN), several organizations operate alternative DNS roots (often referred to as alt roots). ...


Additionally, there are numerous accusations of domain name "front running", whereby registrars, when given whois queries, automatically register the domain name for themselves. Recently, Network Solutions has been accused of this.[7]


Truth in Domain Names Act

In the United States, the "Truth in Domain Names Act" (actually the "Anticybersquatting Consumer Protection Act"), in combination with the PROTECT Act, forbids the use of a misleading domain name with the intention of attracting people into viewing a visual depiction of sexually explicit conduct on the Internet. The Anticybersquatting Consumer Protection Act (also known as Truth in Domain Names Act), a United States federal law enacted in 1999, is part of A bill to amend the provisions of title 17, United States Code, and the Communications Act of 1934, relating to copyright licensing and carriage of broadcast... The PROTECT Act of 2003 authorized fines and/or imprisonment for up to 30 years for U.S. citizens or residents who engage in illicit sexual conduct abroad. ... Internet pornography is pornography that is distributed via the Internet, primarily via websites, peer-to-peer file sharing, or Usenet newsgroups. ...


See also

Dynamic DNS is a system which allows the domain name data held in a name server to be updated in real time. ... In addition to the Internets main DNS root (currently consisting of 13 nominal root nameservers working in agreement with ICANN), several organizations operate alternative DNS roots (often referred to as alt roots). ... This article is a comparison of DNS server software. ... Round robin DNS is a technique in which load balancing is performed by a DNS server instead of a strictly dedicated machine. ... DNS management software is software that runs Domain Name System server clusters. ...

References

  1. ^ How Internet Explorer uses the cache for DNS host entries. Microsoft (2004). Retrieved on 2006-03-07.
  2. ^ Mockapetris, P (November, 1987). RFC1035: Domain Names - Implementation and Specification. Retrieved on 2007-07-31.
  3. ^ RFC 1101 mentions the WKS syndrome as an example of structures and methods which can work but do not because of indifference or errors on the part of system administrators when maintaining the database. As an update to RFC 1035, it classifies WKS as an historical attempt without officially deprecating it.
  4. ^ The term host name is here being used to mean an FQDN for a host, such as eg. en.wikipedia.org., and not just (to use the same example) en .
    While most domain names do indeed designate hosts, some domain name DNS entries may not. In this sense, a (FQDN) hostname is a type of domain name, but not all domain names are actual host names. Cf. this host name vs domain name explanation from the DNS OP IETF Working Group.
  5. ^ McCullagh, Declan (2003-10-03). VeriSign fends off critics at ICANN confab. CNET News.com. Retrieved on 2007-09-22.
  6. ^ Internet Corporation for Assigned Names and Numbers (ICANN). Verisign's Wildcard Service Deployment. Retrieved on 2007-09-22.
  7. ^ [1]

Year 2006 (MMVI) was a common year starting on Sunday of the Gregorian calendar. ... is the 66th day of the year (67th in leap years) in the Gregorian calendar. ... Year 2007 (MMVII) was a common year starting on Monday of the Gregorian calendar in the 21st century. ... is the 212th day of the year (213th in leap years) in the Gregorian calendar. ... A fully qualified domain name (or FQDN) is an unambiguous domain name that specifies the nodes position in the DNS tree hierarchy absolutely. ... A fully qualified domain name (or FQDN) is an unambiguous domain name that specifies the nodes position in the DNS tree hierarchy absolutely. ... An IETF working group, or WG for short, is a working group of the IETF. It operates on rough consensus, is open to all who want to participate, has discussions on an open mailing list, and may hold meetings at IETF meetings. ... Declan McCullagh is a journalist and columnist for CNets news. ... Year 2003 (MMIII) was a common year starting on Wednesday of the Gregorian calendar. ... is the 276th day of the year (277th in leap years) in the Gregorian calendar. ... Year 2007 (MMVII) was a common year starting on Monday of the Gregorian calendar in the 21st century. ... is the 265th day of the year (266th in leap years) in the Gregorian calendar. ... ICANN headquarters ICANN (IPA /aɪkæn/) is the Internet Corporation for Assigned Names and Numbers. ... Year 2007 (MMVII) was a common year starting on Monday of the Gregorian calendar in the 21st century. ... is the 265th day of the year (266th in leap years) in the Gregorian calendar. ...

External links

Paul Vixie is the author of several RFCs and well known UNIX system programs, among them SENDS, proxynet, rtty and Vixie cron. ... The Association for Computing Machinery, or ACM, was founded in 1947 as the worlds first scientific and educational computing society. ...

  Results from FactBites:
 
Domain name system - Wikipedia, the free encyclopedia (4200 words)
Domain names are arranged in a tree, and cut into zones, which are served by nameservers.
The domain name space is a tree of domain names.
Domain names must use only a subset of ASCII characters—the Roman alphabet in upper and lower case, the digits 0 through 9, the dot, and the hyphen.
  More results at FactBites »

 
 

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