"NOS" is the term used to describe a networked environment in which various types of resources, such as user, group, and computer accounts, are stored in a central repository that is controlled and accessible to end users. Typically a NOS environment is comprised of one or more servers that provide NOS services, such as authentication and account manipulation, and multiple end users that access those services.
Microsoft's first integrated NOS environment became available in 1990 with the release of Windows NT 3.0, which combined many features of the LAN Manager protocols and OS/2 operating system. The NT NOS slowly evolved over the next eight years until Active Directory was first released in beta in 1997.
Under Windows NT, the "domain" concept was introduced, providing a way to group resources based on administrative and security boundaries. NT domains are flat structures limited to about 40,000 objects (users, groups, and computers). For large organizations, this limitation imposed superficial boundaries on the design of the domain structure. Often, domains were geographically limited as well because the replication of data between domain controllers (i.e., servers providing the NOS services to end users) performed poorly over high-latency or low-bandwidth links. Another significant problem with the NT NOS was delegation of administration, which typically tended to be an all-or-nothing matter at the domain level.
Microsoft was well aware of these limitations and needed to rearchitect their NOS model into something that would be much more scalable and flexible. For that reason, they looked to LDAP-based directory services as a possible solution.
In generic terms, a directory service is a repository of network, application, or NOS information that is useful to multiple applications or users. Under this definition, the Windows NT NOS is a type of directory service. In fact, there are many different types of directories, including Internet white pages, email systems, and even the Domain Name System (DNS). While each of these systems have characteristics of a directory service, X.500 and the Lightweight Directory Access Protocol (LDAP) define the standards for how a true directory service is implemented and accessed.
In 1988, the International Telecommunication Union (ITU) and International Organization of Standardization (ISO) teamed up to develop a series of standards around directory services, which has come to be known as X.500. While X.500 proved to be a good model for structuring a directory and provided a lot of functionality around advanced operations and security, it was difficult to implement clients to utilize it. One reason is that X.500 is based on the OSI (Open System Interconnection) protocol stack instead of TCP/IP, which had become the standard for the Internet. The X.500 directory access protocol (DAP) was very complex and implemented a lot of features most clients never needed. This prevented large-scale adoption. It is for this reason that a group headed by the University of Michigan started work on a "lightweight" X.500 access protocol that would make X.500 easier to utilize.
The first version of the Lightweight Directory Access Protocol (LDAP) was released in 1993 as RFC 1487, but due to the absence of many features provided by X.500, it never really took off. It wasn't until LDAPv2 was released in 1995 as RFC 1777 that LDAP started to gain popularity. Prior to LDAPv2, the primary use of LDAP was as a gateway between X.500 servers. Simplified clients would interface with the LDAP gateway, which would translate the requests and submit it to the X.500 server. The University of Michigan team thought that if LDAP could provide most of the functionality necessary to most clients, they could remove the middleman (the gateway) and develop an LDAP-enabled directory server. This directory server could use many of the concepts from X.500, including the data model, but would leave out all the overheard provided by the numerous features it implemented. Thus the first LDAP directory server was released in late 1995 by the University of Michigan team, and it turned into the basis for many future directory servers.
In 1997, the last major update to the LDAP specification was described in RFC 2251. It provided several new features and made LDAP robust enough and extensible enough to be suitable for most vendors to implement. Since then, companies such as Netscape, Sun, Novell, and Microsoft have developed LDAP-based directory servers. Most recently, RFC 3377 was released, which summarizes all of the major LDAP RFCs.