Domain Name System (DNS) is a database system that
translates a computer's fully qualified domain name into an
IP address.
Networked computers use IP addresses to locate and connect to each
other, but IP addresses can be difficult for people to remember. For
example, on the web, it's much easier to remember the domain name
www.amazon.com than it is to remember its corresponding
IP address (207.171.166.48). DNS allows you to connect to another
networked computer or remote service by using its user-friendly domain
name rather than its numerical IP address. Conversely, Reverse DNS
(rDNS) translates an IP address into a domain name.
Each organization that maintains a computer network will have at least
one server handling DNS queries. That server, called a name server,
will hold a list of all the IP addresses within its network, plus a
cache of IP addresses for recently accessed computers outside the
network. Each computer on each network needs to know the location of
only one name server. When your computer requests an IP address, one of
three things happens, depending on whether or not the requested IP
address is within your local network:
- If the requested IP address is registered locally (i.e., it's within
your organization's network), you'll receive a response directly from
one of the local name servers listed in your workstation
configuration. In this case, there usually is little or no wait for a
response.
- If the requested IP address is not registered locally (i.e.,
outside your organization's network), but someone within your
organization has recently requested the same IP address, then the
local name server will retrieve the IP address from its cache. Again,
there should be little or no wait for a response.
- If the requested IP address is not registered locally, and you are
the first person to request information about this system in a certain
period of time (ranging from 12 hours to one week), then the local name
server will perform a search on behalf of your workstation. This
search may involve querying two or more other name servers at
potentially very remote locations. These queries can take anywhere
from a second or two up to a minute (depending on how well connected
you are to the remote network and how many intermediate name servers must
be contacted). Sometimes, due to the lightweight protocol used for
DNS, you may not receive a response. In these cases, your workstation
or client software may continue to repeat the query until a response
is received, or you may receive an error message.
When you use an application such as telnet to connect to
another computer, you most likely type in the domain name rather than
the IP address of that computer. The telnet application takes the
domain name and uses one of the above methods to retrieve its
corresponding IP address from the name server. A good analogy is to
think of DNS as an electronic telephone book for a computer
network. If you know the name of the computer in question, the name
server will look up its IP address.
Within most Internet applications, you will not see the IP
address of the computer to which you're connecting.
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