In software engineering, a directory is similar to a dictionary; it enables the look up of a name and information associated with that name. As a word in a dictionary may have multiple definitions, in a directory, a name may be associated with multiple, different, pieces of information. Likewise, as a word may have different parts and different definitions, a name in a directory may have many different types of data. Based on this rudimentary explanation of a directory, a directory service is simply the software system that stores, organizes and provides access to information in a directory.

Directories may be very narrow in scope, supporting only a small set of node types and data types, or they may be very broad, supporting an arbitrary or extensible set of types. In a telephone directory, the nodes are names and the data items are telephone numbers. In the DNS the nodes are domain names or internet addresses. In a directory used by a network operating system, the nodes represent resources that are managed by the OS, including users, computers, printers and other shared resources. Many different directory services have been used since the advent of the Internet but this article focuses mainly on those that have descended from the X.500 directory service.

Introduction

A simple directory service called a naming service maps the names of network resources to their respective network addresses. With the name service type of directory, a user doesn't have to remember the physical address of a network resource; providing a name will locate the resource. Each resource on the network is considered an object on the directory server. Information about a particular resource is stored as attributes of that object. Information within objects can be made secure so that only users with the available permissions are able to access it. More sophisticated directories are designed with namespaces as Subscribers, Services, Devices, Entitlements, Preferences, Content and so on. This design process is highly related to Identity management.

A directory service defines the namespace for the network. A namespace in this context is the term that is used to hold one or more objects as named entries. The directory design process normally has a set of rules that determine how network resources are named and identified. The rules specify that the names be unique and unambiguous. In X.500 (the directory service standards) and LDAP the name is called the distinguished name (DN) and is used to refer to a collection of attributes (relative distinguished names) which make up the name of a directory entry.

A directory service is a shared information infrastructure for locating, managing, administering, and organizing common items and network resources, which can include volumes, folders, files, printers, users, groups, devices, telephone numbers and other objects. A directory service is an important component of a NOS (Network Operating System). In the more complex cases a directory service is the central information repository for a Service Delivery Platform. For example, looking up "computers" using a directory service might yield a list of available computers and information for accessing them.