Active Directory Overview and its Components

ACTIVE DIRECTORY (OVERVIEW)

Keeping track of everything on your network is a time-consuming task. Even on small networks, users tend to have difficulty finding network file and printer shares. Without some kind of network directory, medium and large networks are impossible to manage, and users will often have a difficult time finding resources on the network.

Previous versions of Microsoft Windows included services to help users and administrators find network resources. Network Neighborhood is useful in many environments, but users often complain about the clumsy interface, and its unpredictability baffles many administrators. The WINS Manager and Server Manager could be used to view a list of systems on the network, but they were not readily available to end users. Administrators utilized User Manager to add and delete users, an entirely different type of network object. These applications got the job done, but proved to be inefficient—especially in large networks.

All of these objects resided in a common container: the Microsoft Windows NT domain. Windows NT domains worked best in small-sized and medium-sized environments. Administrators of large environments were forced to partition their network into multiple domains interconnected with trusts. Microsoft Windows 2000 Server introduces Active Directory to replace domain functionality. Active Directory will continue to get the job done, but in a much more efficient way. Active Directory can be replicated between multiple domain controllers, so no single system is critical. In this way, the crucial data stored within Active Directory is both redundant and load-balanced.

A directory, in the most generic sense, is a comprehensive listing of objects. A phone book is a type of directory that stores information about people, businesses, and government organizations. Phone books typically record names, addresses, and phone numbers. Active Directory is similar to a phone book in several ways, and it is far more flexible. Active Directory will store information about organizations, sites, systems, users, shares, and just about any other network object that you can imagine. Not all objects are as similar to each other as those stored in the phone book, so Active Directory includes the ability to record different types of information about different objects. This chapter will teach you

• What Active Directory is
• How standard protocols like DNS dynamic update protocol and Lightweight Directory Access Protocol (LDAP) are used
• How to plan for migrating to Active Directory
• What objects, schema, object classes, and attributes are
• How replication and partitioning work
• What the global catalog is useful for and how to use it.

Active Directory Components

As I mentioned in the introduction, Active Directory stores information about network components. It allows clients to find objects within its namespace. The term namespace (also known as console tree) refers to the area in which a network component can be located. For example, the table of contents of this book forms a namespace in which chapters can be resolved to page numbers. DNS is a namespace that resolves host names to IP addresses. Telephone books provide a namespace for resolving names to telephone numbers. Active Directory provides a namespace for resolving the names of network objects to the objects themselves. Active Directory can resolve a wide range of objects, including users, systems, and services on a network.

Everything that Active Directory tracks is considered an object. An object is any user, system, resource, or service tracked within Active Directory. The generic term object is used because Active Directory is capable of tracking a variety of items, and many objects can share common attributes.

Attributes describe objects in Active Directory. For example, all User objects share attributes to store a user name, full name, and description. Systems are also objects, but they have a separate set of attributes that include a host name, an IP address, and a location.

The set of attributes available for any particular object type is called a schema. The schema makes object classes different from each other. Schema information is actually stored within Active Directory, which allows administrators to add attributes to object classes and have them distributed across the network to all corners of the domain, without restarting any domain controllers.

A container is a special type of object used to organize Active Directory. It does not represent anything physical, like a user or a system. Instead, it is used to group other objects. Container objects can be nested within other containers.

Each object in an Active Directory has a name. These are not the names that you are accustomed to, like "Tony" or "Eric." They are LDAP distinguished names. LDAP distinguished names are complicated, but they allow any object within a directory to be identified uniquely regardless of its type. 

The term tree is used to describe a set of objects within Active Directory. When containers and objects are combined hierarchically, they tend to form branches—hence the term. A related term is contiguous subtree, which refers to an unbroken branch of the tree.

Continuing the tree metaphor, the term forest describes trees that are not part of the same namespace but that share a common schema, configuration, and global catalog. Trees in a forest all trust each other, so objects in these trees are available to all users if the security allows it. Organizations that are divided into multiple domains should group the trees into a single forest.

A site is a geographical location, as defined within Active Directory. Sites correspond to logical IP subnets, and as such, they can be used by applications to locate the closest server on a network. Using site information from Active Directory can profoundly reduce the traffic on wide area networks.