Directory services
Directory Services identify all the resources on a network and make them accessible to users and applications. Directory Services typically catalogue organisations or people. Data stored by such services include email addresses, unique identifiers (such as Web addresses or ISBN numbers), or listings of domain names servers, computers, databases, printers, modems, scanners, and mail folders. Directory Services also handle security processes such as authentication and authorisation.
If possible, the directory service should make transparent the physical network topology and protocols, the set of rules governing the exchange or transmission of data electronically between devices. A user on a network can then access any resource without knowing where or how it is physically connected.
LDAP, NDS and X.500 ITU
There are a number of directory services that are used widely. Two of the most important ones are LDAP, (Lightweight Directory Access Protocol) which is used primarily for e-mail addresses, and Netware Directory Service (NDS), which is used on Novell Netware networks. Most directory services are based on the X.500 ITU standard.