DNS domain and hostnames

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.your_company.COM., the host, volga, appears in the domain your_company.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 your_company.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.your_company.COM.). A domain name minus the dot may be seen as a relative domain name (example: volga.your_company). 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).