Domain Name Space
            The Internet Domain Name System (DNS) is a set of distributed databases containing IP addresses and their corresponding domain names.  Each domain name is mapped to a particular numeric address.  DNS, with servers located all over the Internet, performs the translation back and forth between names and numbers.  This scheme enables users to invoke an easy-to-remember name (e.g. www.google.co.in) instead of a more mysterious string of numbers (e.g. 160.96.7.25) when using the Internet.  In the beginning, the original set of machine names used throughout the Internet formed a flat name space.  Hence, a hierarchical naming scheme is used and the name system is decentralized by delegating authority for parts of the namespace and distributing responsibility for the mapping between names and addresses.
Valid Domain Names
            Domain names can only contain alphabets, numbers and dashes (i.e. hyphens or the minus signs).  The underscore character, “_”, is an invalid character.  SGNIC Authority will reject applications for domain names containing invalid characters. Each field in the domain name should be less than sixty three characters long.
            Domain names and its Sub domain names are unique.

Definition of FQDN (Fully Qualified Domain Name)
            The domain names always read from the node up to the root.  Hence the last label is the label of the root (null).
If a label is terminated by a null string, then it is called fully qualified domain (FQDN) that contains the full name of a host.
Example: CS.princeton.edu
Definition of PQDN
            If a label is not terminated by a null string, then it is called as partially qualified domain name (PQDN).  It starts from the node, but it does not reach the root.
            Example: CS

• A domain is sub tree of the domain name space.  A domain may be divided into sub domain.