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 lowest level at which administrative powers can be delegated.
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.
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.