DNS

The Domain Name system (DNS) associates various sorts of information with so-called 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. 208.77.150.31, 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.

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.

Preeminently, 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 "www.abcxyz.com") which is rather easier to remember than the IP address 208.77.150.31. People take advantage of this when they recite meaningful URLs and e-mail addresses without caring how the machine will actually locate them.



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.

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).


PROTOCOL DETAILS

DNS primarily uses UDP on port 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 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.

EXTENSIONS TO DNS

EDNS is an extension of the DNS protocol which enhances the transport of DNS data in UDP packages, and adds support for expanding the space of request and response codes.



THE INTERNET DOMAIN STRUCTURE

The Internet domain name space is divided into several organizational and geographical top-level domains. Top-level non-geographic domains include:

COM
commercial organizations

EDU
educational organizations

GOV
government organizations

INT
international organizations

MIL
military departments

NET
networking organizations

ORG
miscellaneous organizations
The top-level domains are shown here in upper case. In fact, domain names are case-insensitive. That is, the COM domain is equivalent to the com domain and is usually written in lower case.

Although the three-letter top-level domains are primarily intended for use by companies and organizations in the United States, domain names in COM, EDU, INT, NET, and ORG are often allocated to companies and organizations in other countries.

Geographical top-level domains generally follow the ISO 3166 standard, which contains abbreviations for each country (for example, ca for Canada, ch for Switzerland, and it for Italy). An exception is the domain uk (United Kingdom) which is used for Great Britain rather than the value gb defined by the standard.