What Is a Directory?

Simply put, a directory is a container of data, like the company directory I used to use. Another directory is TV Guide, which lists television programs and the times they are shown. These traditional directories are printed and distributed at regular intervals. They do not change but are replaced by a new issue; therefore, they can be considered off-line directories. Off-line directories are generally used for publishing information that is read-only.

An online directory is a directory that can be accessed and updated electronically over a computer network—whether that network is a local area network (LAN), wide area network (WAN), or even the Internet. Many off-line directories have an online, or electronic, counterpart. Phone companies publish their listings and yellow pages on the Web and provide an easy-to-use interface.

Other types of online directories include application directories and purpose–specific directories. An application directory is one that is tied to a software application—such as Lotus Notes or Novell GroupWise. Both use proprietary directories that are tailored to the specific needs of the application.

A purpose-specific directory can be used by any application, but it is tied to holding data for a narrow purpose. A good example of this type of directory is the Domain Name System (DNS) used by the Internet. While many different applications use DNS, the information contained within it is only useful for specific tasks—name and IP address resolution.

Network directories are online directories that store information about network resources and services. Generally, this information includes user information, security data, and lists of services available, such as printer and publishing services.

Active Directory is considered a network directory, but it has been designed to allow other data to be stored in it. For example, Microsoft has implemented the Dynamic DNS service of Windows 2000 in Active Directory. By using a standardized data model and programming interface, known as LDAP (more on that later), the directory information can be used by any application. Applications can even modify the data model to contain new classes of information for a specific purpose.