Domain name
The Domain Name System, most often known as simply DNS, is a core feature of the Internet. It is a distributed database that handles the mapping between host names (domain names), which are more convenient for humans, and the numerical IP address, which a computer can use directly. A domain name such as www.wikipedia.org is e.g. part of a URL such as http://www.wikipedia.org/wiki/Train and part of an e-mail address such as apache@www.wikipedia.org.
130.94.122.199 is the corresponding numerical internet address. The domain name system acts much like an automated phone book, so you can "call" www.wikipedia.org instead of 130.94.122.199.
So, it converts human-friendly names such as "www.wikipedia.org" into computer-friendly (IP) addresses such as 130.94.122.199.
In spite of the similarity in notation of IP addresses and domain names, both having parts separated by dots, the hierarchy in an IP address is from left to right and in a domain name from right to left.
The domain name consists of two or more parts. From right to left each part specifies a further subdivision, with at the right (the root) the top-level domain, e.g. org or nl. A second-level domain consists of two parts, e.g. wikipedia.org. When one is the registered owner of it, one is also authorised to use domain names such as nl.wikipedia.org (a third level domain) and some.other.stuff.wikipedia.org (a fifth level domain).
Many domain names have www at the left, but not necessarily. If not, they usually lead to the same webpage as with "www." in front, e.g. ns.nl and www.ns.nl. However, "www." can not always be omitted. The shortest domain name (and hence the shortest URL) is p.ro. Some domain names have "www" not as the third but the second part from the right, e.g. www.com, www.ru, www.kz.
DNS was first invented in 1983 by Paul Mockapetris; the original specifications are described in RFC 882. In 1987 RFC 1034 and RFC 1035 were published which updated the DNS specification and made RFC 882 and RFC 883 obsolete. Subsequent to that there have been quite a few RFCs published that propose various extensions to the core protocols.
Delegation
DNS implements a hierarchical name space by allowing name service for parts of a name space known as zones to be "delegated" by a name server to subsidiary name-servers. DNS also provides additional information, such as alias names for systems, contact information, and which hosts act as mail hubs for groups of systems or domains.
The present restriction on the length of domain names is 63 characters, excluding the www. and .com or other extension. Domain names are also limited to a subset of ASCII characters, preventing many languages from representing their names and words correctly.
The Punycode-based IDNA system, which maps Unicode strings into the valid DNS character set, has been approved by ICANN and adopted by some registries as a workaround.
The DNS system is run by various flavors of DNS software, including:
- BIND (Berkeley Internet Name Domain), the most commonly used namedaemon.
- DJBDNS (Dan J Bernstein's DNS implementation)
- MaraDNS
- NSD (Name Server Daemon)
- PowerDNS
DNS uses TCP and UDP ports 53 to serve requests.
The Internet DNS system
Any IP computer network can use DNS to implement its own private name system. However, the term "domain name" is most commonly used to refer to domain names implemented in the public Internet DNS system.
This is based on thirteen "root servers" worldwide, whose IP addresses are distributed independently of the DNS using a "root hints" file. From these root servers, the rest of the Internet DNS name space is delegated to other DNS servers which serve names within specific parts of the DNS name space.
All but three of the root servers are, at least nominally, in the United States of America. However, because many root servers are actually implemented using 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 are now outside the United States.
An 'owner' of a domain name can be found by looking in the whois database: for most gTLDs a basic WHOIS is held by ICANN, with the detailed WHOIS maintained by the domain registry which controls that domain.
For the 240+ Country Code TLDs the position is usually that the registry holds the entire authorative WHOIS for that extension, as part of their many functions.
Politics
The current way the main DNS system is controlled is often criticized.
The most common problems pointed at are that it is abused by monopolies or near-monopolies such as VeriSign Inc., and problems with assignment of top-level domains.
Criticisms
Some also allege that many implementations of DNS server software fail to work gracefully with dynamically allocated IP addresses, although that is the failure of specific implementations and not failures of the protocol itself.
See also: cybersquatting, dynamic DNS, ICANN, DNSSEC
External links
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