In hierarchical namespace, NAMES are DEFINED in an inverted­tree STRUCTURE with the root at the top. The tree can have only 128 levels: level 0 (root) to level 127. Each node in the tree has a label, which is a string with a maximum of 63 characters. The root label is a null string (EMPTY string). In this tree, all the labels have a different name, which GUARANTEE the uniqueness of the domain names. A full domain name is a sequence of labels separated by dots (.). The domain names are always read from the node up to the root.
● Fully Qualified Domain Name 
If a label is terminated by a null string or empty string, it is called a fully qualified domain name (FQDN).
● Partially Qualified Domain Name 
If a label is not terminated by a null string, it is called a partially qualified domain name (PQDN). A PQDN starts from a node, but it does not end with the root.

In hierarchical namespace, names are defined in an inverted­tree structure with the root at the top. The tree can have only 128 levels: level 0 (root) to level 127. Each node in the tree has a label, which is a string with a maximum of 63 characters. The root label is a null string (empty string). In this tree, all the labels have a different name, which guarantee the uniqueness of the domain names. A full domain name is a sequence of labels separated by dots (.). The domain names are always read from the node up to the root.
● Fully Qualified Domain Name 
If a label is terminated by a null string or empty string, it is called a fully qualified domain name (FQDN).
● Partially Qualified Domain Name 
If a label is not terminated by a null string, it is called a partially qualified domain name (PQDN). A PQDN starts from a node, but it does not end with the root.