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Active Directory first introduced with Windows 2000 server. Active Directory allows administrators to create a more flexibility Network structure. Active Directory is a Directory services. Active Directory services are based approach to Network design it allows for large distributed network environment that have a common centralized authority for network security.

2> Active Directory provides a single point of management for Windows- based user account clients and application.

Domain tree: - In Windows 2000, a domain can be a child of another domain (e.g., child.domain.com is a child of domain.com). A child domain name always includes the complete parent domain name. A child domain and its parent share a two-way transitive trust. A domain tree exists when one domain is the child of another domain. A domain tree must have a contiguous namespace

Domain forest: - A forest is a collection of trees that don’t necessarily form a contiguous namespace (although each tree must be contiguous). This arrangement might be useful if your company has multiple root DNS addresses.Each forest has admin Group. Members of the group have authority over all the Domain trees in the forest .Each domain has a Domain admin
Group and administrator in a parent Domain automatically transitive trust relationship .This type of structure is known as a hierarchical structure

Why we need to Install Active Directory for Exchange server.

Exchange server is fully integrated with AD Exchange server Is a Prerequisite, it will not install unless AD is there.