DNS domain names are structured as a listing,
from left to right, of the
domain name space from the host to the root. For example, in the DNS
name volga.mynet.COM., the host, volga, appears in the
domain mynet.COM, which is a subdomain of COM,
which is a domain under the root.
Specifying the root domain is optional; when indicated it is shown as a
trailing dot (.).
This naming structure is similar to UNIX filenames in the following ways:
location
Each name gives its location in a tree. For example: the filename
/usr/bin/message and the domain name mynet.COM.:
uniqueness
Each name need only be unique within its portion (domain or directory)
of the tree. For example: two files named tcl and two hosts
named volga:
relativity
Each name can be relative or absolute (fully qualified).
A fully qualified domain name ends at the root domain, indicated by a
trailing dot (example: volga.mynet.COM.). A domain name minus
the dot may be seen as a relative domain name
(example: volga.mynet). Again, parallels exist with the
UNIX filesystem, in which the file /usr/bin/tcl may be
indicated as /usr/bin/tcl (its absolute name) or
simply tcl (its relative name).