Domain
Names System - DNS
The Domain Names System or DNS is a system that stores
information about hostnames and domain names in a kind
of distributed database on networks, such as the Internet.
Most importantly, it provides a physical location (IP
address) for each hostname, and lists the mail exchange
servers accepting e-mail for each domain.
The DNS provides a vital service on the Internet as
it allows the transmission of technical information
in a user-friendly way. While computers and network
hardware work with IP addresses to perform tasks such
as addressing and routing, humans generally find it
easier to work with hostnames and domain names in URLs
and e-mail addresses. The DNS therefore mediates between
the needs and preferences of humans and of software.
How the DNS works in theory
Meet the players
The practical operation of the DNS system consists
of three players:
* The DNS resolver, a DNS client program which runs
on a user's computer and generates DNS requests on behalf
of software programs;
* The recursive DNS server, which searches through the
DNS in response to queries from resolvers and returns
answers to those resolvers;
and,
* The authoritative DNS server which hands out answers
to queries from recursors, either in the form of an
answer, or in the form of a delegation (i.e. referral
to another authoritative DNS server).
Understanding the parts of a domain name
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 en.wikipedia.org has the top-level
domain org).
* Each label to the left specifies a subdivision or
subdomain of the domain above it. Note that "subdomain"
expresses relative dependence, not absolute dependence:
for example, wikipedia.org comprises a subdomain of
the org domain, and en.wikipedia.org could form a subdomain
of the domain wikipedia.org (in practice, however, en.wikipedia.org
actually represents a hostname - see below). 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 255 characters. But in practice some domain registries
have shorter limits than that.
* Finally, the leftmost part of the domain name (usually)
expresses the hostname. The rest of the domain name
simply specifies a way of building a logical path to
the information required; the hostname is the actual
target system name for which an IP address is desired.
For example, the domain name en.wikipedia.org has the
hostname "en".
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
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 servers: the servers to query when looking
up (resolving) a top-level domain name.
|
.