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The Domain Name System or DNS is a system that stores information about host names and domain names in a kind of distributed database on networks, such as the Internet. Most importantly, it provides an IP address for each host name, and lists the mail exchange servers accepting e-mail for each domain.

The DNS forms a vital part of the Internet, because in contrast to hardware that requires only IP addresses to perform its tasks such as address routing, humans use host names and domain names, for example in URLs and e-mail addresses.

Paul Mockapetris invented the DNS in 1983; the original specifications appear in RFC 882. In 1987 the publication of RFC 1034 and RFC 1035 updated the DNS specification and made RFC 882 and RFC 883 obsolete. Several more recent RFCs have proposed various extensions to the core protocols.

1 How the DNS works

A domain name usually consists of two or more parts (technically labels) separated by dots. The rightmost label conveys the top-level domain (for example, the address www.wikipedia.org has the top-level domain org). Each label to the left specifies a subdivision or subdomain (for example, wikipedia.org is a subdomain of org and www.wikipedia.org is a subdomain of wikipedia.org). In theory, this subdivision can go down to 127 levels deep, and each label can contain up to 63 characters, as long as the whole domain name does not exceed a total length of 254 characters. But in practice some domain registriesIn the domain name system on the Internet there is a need for databases to be kept of which domain name maps to which IP address. A registry has two main tasks: # giving out domain names under their top level domain to those who ask for them; and # making have shorter limits than that.

The DNS 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. The hierarchy of authoritative DNS servers matches the hierarchy of domains. At the top of the hierarchy stand the root servers: the servers to query when looking up (resolving) a top-level domain name.

An example may clarify this. Suppose an application needs to find the IP address of www.wikipedia.org. Before starting, the local system has to know where to find the root serversA root nameserver is a DNS server that answers requests for the root namespace domain, and redirects requests for a particular top-level domain to that TLD's nameservers. All domain names on the Internet actually end in a . period) character that is, tech. It starts by asking one of these root servers -- for example, the server with the IP address "198.41.0.4". The root server replies with a delegation meaning roughly, "I don't know the address of www.wikipedia.org, but I do know that the DNS server at 204.74.112.1 has information on the org domain." The local DNS client then asks that DNS server, which replies, "I don't know the address of www.wikipedia.org, but I do know that the DNS server at 207.142.131.234 has information on the wikipedia.org domain." Finally the request goes to this third DNS server (207.142.131.234), which replies with the required IP address. The whole process thus utilises recursive searchingIn mathematics and computer science, recursion is a particular way of specifying (or constructing) a class of objects (or an object from a certain class) with the help of a reference to other objects of the class: a recursive definition defines objects in.

When an application (such as a web browser), wants to find the IP address of a domain name, it doesn't necessarily follow all these steps. The web browser will contact a DNS cache, which provides the base of the Domain Name System. The DNS cache will receive a query for a domain, follow all the steps needed to find the IP, as described above, and only return the final result to the application. However, it will also remember all the results for a period of time, thereby speeding up queries that overlap and repeated queries. When many people use the same DNS cache, chances of finding the results already in the cache increase, and often subsequent queries generate no external network calls at all. Organisations or Internet service provider commonly run a DNS cache for all their users. A TTLTime to live TTL is an 8-bit field in the Internet Protocol (IP) header that indicates how many more hops this packet should be allowed to make before being discarded or returned. It is the 9th octet of 20 in the IP header. TTL's also occur in the Domain value specified by the authoritative name servers governs the maximum length of time a DNS cache may remember the request results.

The system outlined above provides a somewhat simplified scenario. The DNS includes several other functions:

Host names and IP addresses do not necessarily match on a one-to-one basis. Many host names may correspond to a single IP address: combined with virtual hostingVirtual hosting is a method that web servers use to host more than one domain name on the same computer and IP address. With web browsers that support HTTP/1. 1 (as most do), upon connecting to a webserver, they send the address that the user typed into t, this allows a single machine to serve many web sites. Alternatively a single host name may correspond to many IP addresses: this can facilitate fault toleranceIn 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. Fundamental mechanisms for making a computer system fault-toleran 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 agentsA 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. The MTA works behind 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.
To provide resilience in the event of computer failure, multiple DNS servers provide coverage of each domain. In particular, thirteen root servers exist worldwide. DNS programs or operating systems have the IP addresses of these servers built in. The USA hosts, at least nominally, all but three of the root servers. However, because many root servers actually implement anycast, where many different computers can share the same IP address to deliver a single service over a large geographic region, most of the physical (rather than nominal) root servers now operate outside the USA.

The DNS uses TCP and UDP ports 53 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 is typically used only when the response data size exceeds 512 bytes, or for such tasks as AXFR.

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