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Active Directory is an implementation of LDAP directory services by Microsoft for use in Windows environments. Active Directory allows administrators to assign enterprise-wide policies, deploy programs to many computers, and apply critical updates to an entire organization. An Active Directory stores information and settings relating to an organization in a central, organized, accessible database. Active Directory networks can vary from a small installation with a few hundred objects, to a large installation with millions of objects.

Active Directory was previewed in 1996, released first with Windows 2000, and saw some revision to extend functionality and improve administration in Windows Server 2003.

Active Directory was called NTDS (NT Directory Service) in older Microsoft documents. This name remains in some AD binaries as well.

Contents [hide]

1 Structure

1.1 Objects

1.2 Forests, trees, and domains

1.3 Physical structure and replication

2 Naming

3 Trust

3.1 Trusts in Windows 2000 (native mode)

4 ADAM

5 Alternatives

6 See also

7 Notes

8 External links

[edit] Structure

[edit] Objects

Active Directory is a directory service used to store information about the network resources across a domain.

An Active Directory (AD) structure is a hierarchical framework of objects. The objects fall into three broad categories -- resources (e.g. printers), services (e.g. e-mail), and users (accounts, or users and groups). The AD provides information on the objects, organizes the objects, controls access, and sets security.

Each object represents a single entity -- whether a user, a computer, a printer, an application, or a shared data source--and its attributes. Objects can also be containers of other objects. An object is uniquely identified by its name and has a set of attributes--the characteristics and information that the object can contain--defined by a schema, which also determines the kind of objects that can be stored in the AD.

Each attribute object can be used in several different schema class objects. These schema objects exist to allow the schema to be extended or modified when necessary. However, because each schema object is integral to the definition of AD objects, deactivating or changing these objects can have serious consequences because it will fundamentally change the structure of AD itself. A schema object, when altered, will automatically propagate through Active Directory and once it is created it can only be deactivated -- not deleted. Changing the schema usually requires a fair amount of planning.[1]

[edit] Forests, trees, and domains

The framework that holds the objects is viewed at a number of levels. At the top of the structure is the Forest - the collection of every object, its attributes and rules (attribute syntax) in the AD. The forest holds one or more transitive, trust-linked Trees. A tree holds one or more Domains and domain trees, again linked in a transitive trust hierarchy. Domains are identified by their DNS name structure, the namespace. A domain has a single DNS name.

The objects held within a domain can be grouped into containers called Organizational Units (OUs). OUs give a domain a hierarchy, ease its administration, and can give a semblance of the structure of the AD's company in organizational or geographical terms. OUs can contain OUs - indeed, domains are containers in this sense - and can hold multiple nested OUs. Microsoft recommends as few domains as possible in AD and a reliance on OUs to produce structure and improve the implementation of policies and administration. The OU is the common level at which to apply group policies, which are AD objects themselves called Group Policy Objects (GPOs), although policies can also be applied to domains or sites (see below). The OU is the level at which administrative powers are commonly delegated, but granular delegation can be performed on individual objects or attributes as well.

As a further subdivision AD supports the creation of Sites, which are physical, rather than logical, groupings defined by one or more IP subnets. Sites distinguish between locations connected by low-speed (e.g. WAN, VPN) and high-speed (e.g. LAN) connections. Sites can contain one or more domains and domains can contain one or more sites. This is important to control network traffic generated by replication and to refer clients to the nearest domain controllers.

The actual division of the company's information infrastructure into a hierarchy of one or more domains and top-level OUs is a key decision. Common models are by business, by geographical location, or by IT roles. These models are also often used in combination, but Microsoft recommends that OUs be structured to facilitate administrative delegation and group policy application.

[edit] Physical structure and replication

Physically the AD information is held on one or more equal peer domain controllers (DCs), replacing the NT PDC/BDC format (although there is a 'more equal' flexible single master operation (FSMO) server for some operations, which can simulate a PDC). Each DC holds a single domain partition and a read-and-write copy of the AD; changes on one computer being synchronized (converged) between all the DC computers by multi-master replication. Servers without AD are called Member Servers.

Unlike earlier versions of Windows which used NetBIOS to communicate, Active Directory is fully integrated with DNS and TCP/IP -- indeed DNS is required. To be fully functional, the DNS server must support SRV resource records or service records.

AD replication is 'pull' rather than 'push'. The Knowledge Consistency Checker (KCC) creates a replication topology of site links using the defined sites to manage traffic. Intrasite replication is frequent and automatic as a result of change notification, which triggers peers to begin a pull replication cycle. Intersite replication intervals are less frequent and do not use change notification,