Directory services have existed in one form or another since the early days of computing to provide basic lookup and authentication functionality for enterprise network implementations. A directory service provides detailed information about a user or object in a network, much in the same way that a phone book is used to look up a telephone number for a provided name. For example, a user object in a directory service can store the phone number, email address, department name, and as many other attributes as an administrator desires.
Directory services are commonly referred to as the white pages of a network. They provide user and object definition and administration. Early electronic directories were developed soon after the invention of the digital computer and were used for user authentication and to control access to resources. With the growth of the Internet and the increase in the use of computers for collaboration, the use of directories expanded to include basic contact information about users. Examples of early directories included MVS PROFS (IBM), Grapevine’s Registration Database, and WHOIS.
Application-specific directory services soon arose to address the specific addressing and contact-lookup needs of each product. These directories were accessible only via proprietary access methods and were limited in scope. Applications utilizing these types of directories were programs such as Novell GroupWise, Lotus Notes, and the UNIX sendmail /etc/aliases file.
The further development of large-scale enterprise directory services was spearheaded by Novell with the release of Novell Directory Services (NDS) in the early 1990s. It was adopted by NetWare organizations and eventually was expanded to include support for mixed NetWare/NT environments. The flat, unwieldy structure of NT domains and the lack of synchronization and collaboration between the two environments led many organizations to adopt NDS as a directory service implementation. It was these specific deficiencies in NT that Microsoft addressed with the introduction of AD DS.
The development of the Lightweight Directory Access Protocol (LDAP) corresponded with the growth of the Internet and a need for greater collaboration and standardization. This nonproprietary method of accessing and modifying directory information that fully utilized TCP/IP was determined to be robust and functional, and new directory services implementations were written to utilize this protocol. AD DS itself was specifically designed to conform to the LDAP standard.
Source of Information : Sams - Windows Server 2008 R2 Unleashed