Domain System Network software generally needs a 32-bit Internet address in
order to open a connection or send a datagram. However users prefer to deal with
computer names rather than numbers. Thus there is a database that allows the
software to look up a name and find the corresponding number. When the Internet
was small, this was easy. Each system would have a file that listed all of the
other systems, giving both their name and number. There are now too many
computers for this approach to be practical. Thus these files have been replaced
by a set of name servers that keep track of host names and the corresponding
Internet addresses. (In fact these servers are somewhat more general than that.
This is just one kind of information stored in the domain system.) Note that a
set of interlocking servers are used, rather than a single central one. There
are now so many different institutions connected to the Internet that it would
be impractical for them to notify a central authority whenever they installed or
moved a computer. Thus naming authority is delegated to individual institutions.
The name servers form a tree, corresponding to institutional structure. The
names themselves follow a similar structure. A typical example is the name
BORAX.LCS.MIT.EDU. This is a computer at the Laboratory for Computer Science (LCS)
at MIT. In order to find its Internet address, you might potentially have to
consult 4 different servers. First, you would ask a central server (called the
root) where the EDU server is. EDU is a server that keeps track of educational
institutions. The root server would give you the names and Internet addresses of
several servers for EDU. (There are several servers at each level, to allow for
the possibly that one might be down.) You would then ask EDU where the server
for MIT is. Again, it would give you names and Internet addresses of several
servers for MIT. Generally, not all of those servers would be at MIT, to allow
for the possibility of a general power failure at MIT. Then you would ask MIT
where the server for LCS is, and finally you would ask one of the LCS servers
about BORAX. The final result would be the Internet address for
BORAX.LCS.MIT.EDU. Each of these levels is referred to as a "domain".
The entire name, BORAX.LCS.MIT.EDU, is called a "domain name". (So are
the names of the higher-level domains, such as LCS.MIT.EDU, MIT.EDU, and EDU.)
Fortunately, you don't really have to go through all of this most of the time.
First of all, the root name servers also happen to be the name servers for the
top-level domains such as EDU. Thus a single query to a root server will get you
to MIT. Second, software generally remembers answers that it got before. So once
we look up a name at LCS.MIT.EDU, our software remembers where to find servers
for LCS.MIT.EDU, MIT.EDU, and EDU. It also remembers the translation of
BORAX.LCS.MIT.EDU. Each of these pieces of information has a "time to
live" associated with it. Typically this is a few days. After that, the
information expires and has to be looked up again. This allows institutions to
change things. The domain system is not limited to finding out Internet
addresses. Each domain name is a node in a database. The node can have records
that define a number of different properties. Examples are Internet address,
computer type, and a list of services provided by a computer. A program can ask
for a specific piece of information, or all information about a given name. It
is possible for a node in the database to be marked as an "alias" (or
nickname) for another node. It is also possible to use the domain system to
store information about users, mailing lists, or other objects. There is an
Internet standard defining the operation of these databases, as well as the
protocols used to make queries of them. Every network utility has to be able to
make such queries, since this is now the official way to evaluate host names.
Generally utilities will talk to a server on their own system. This server will
take care of contacting the other servers for them. This keeps down the amount
of code that has to be in each application program. The domain system is
particularly important for handling computer mail. There are entry types to
define what computer handles mail for a given name, to specify where an
individual is to receive mail, and to define mailing lists.