What is Active Directory

Originally created in the year 1996, Active Directory is usually termed as an AD, was initially utilized with Windows 2000 Server as a directory service for Windows domain networks. Actually, Active Directory is a database that is mainly used for serving a central location for authenticating and authorizing all the users and computers within a network. Besides this, Active Directory make use of the Lightweight Directory Access Protocol (LDAP), which is more like a application protocol used for accessing and preserving information of directory distributed over an IP network.
What is Active Directory?
The fundamental internal structure of the Active Directory consists of array featuring objects that are arranged in a hierarchy. In fact, these objects can be easily categorized generally into resources and security principles. Some of the examples of Active Directory objects are users, computers, groups, sites, services, printers, etc. Every single object is generally considered as an entity featured with particular set of attributes. The attributes of Objects and the type of objects that can be stored in the AD are described by a Schema.
The inherent structure of Active Directory is further categorized into a number of levels that actually depends upon the visibility of objects. An AD network can be organized in four various forms of container structure specifically, Forest, Domains, Organizational Units and Sites.

• Forests:It is a set of Active Directory objects, their attributes and collection of attribute syntax.
• Domain:It is more like a compilation of computers objects in the AD which share a very common set of policies, a name and a database of their members.
• Organizational Units:OUs are containers in which domains are grouped. Their key function is of creating a hierarchy for the domain to resemble the structure of the Active Directory's company in organizational terms.
• Sites: Sites are not dependent on domains and OU structure and therefore more considered as physical groups that is defined by one of more IP subnets. Their key role is of distinguishing between locations connected by low- and high-speed connections.

Mainly, AD has three different levels or logical divisions viz., Forest, Tree and Domain. A Domain is at the lowest stage of a complete network. More importantly it is recognized by its DNS (Domain Name Structure). Always keep in mind that a Tree is just a set of one of more domains in a network while a Forest is more like an assortment of Trees that shares a common global catalog, directory configuration, directory schema and logical structure. Forest is at the highest level of the logical structure and it mainly corresponds to the security boundaries within which the AD objects are accessible.
Within a domain, all the objects that are present are grouped in Organizational Units or OUs. This grouping helps in simplifying the administrative tasks. With OUs, a domain can be divided in a hierarchical way to look like the managerial or departmental structure of an organization. Perhaps, it won't be wrong to consider organizational units as containers which can hold other OUs of the domain.
Group Policies in the form of Group Policy Objects (GPOs) are generally applied to the OUs and administrative powers are also delegated at the OUs.
Moreover, it would be wrong to consider sites as logical structure. They are actually physical groupings used to control network traffic caused due to Active Directory replication. In addition, sites are used to refer the clients to the nearest domain through a Domain Controller (DC). All the information that is present in the Active Directory is actually held in one more domain controller. Each DC has a copy of the Active Directory and every time even minor changes take place in any server, the information gets replicated in all the DC that too contains a copy of the Active Directory. This procedure is termed as Active Directory Replication. Replication in the Active Directory is triggered almost every time an Object is created, removed, moved or adapted.