Since directory services are provided by different vendors, each directory may have a different way of organizing and storing entries.
A directory has either a flat structure or a hierarchical structure. A hierarchical structure allows related entries to be combined into distinct logical groupings that descend from a parent entry. In a flat structure, all entries in the directory are in one logical grouping.
A hierarchical structure can be thought of as an inverted tree. The “root” entry is at the top and is the “ancestor” of all other entries. “Parent” entries represent logical groupings of related entries. If an entry is the parent of no other entry, it is called a “leaf” entry.
In any directory structure, each entry has a fully qualified name that uniquely identifies the entry. Entries also have a common name that is unique only among entries that have the same parent node.
In a hierarchical directory structure, names must contain navigation information. Only at the root node are the common name and the fully qualified name the same. For any other entry, the fully qualified name is constructed by combining the entry’s common name with the fully qualified name of the entry’s parent node.
In a flat directory structure, there is no root node, and every entry’s fully qualified name is the same as its common name.
The Sybase interfaces file is an example of a flat directory. Most network-based directory services provide a hierarchical directory.
