For example, a user object has, among other things, a
first name, last name, logon name, e-mail address, and password. If you??™re familiar with
databases, you should already be familiar with the term schema since a database schema refers
to the structure of the database in the same way the Active Directory schema defines
the Active Directory??™s structure.
If you think of Active Directory as a database, then naturally you would expect it to
have an index. This is called a global catalog (GC), and it stores a subset of the information
regarding each object that you can use to search the directory. The information in the GC
gets replicated to domain controllers in different sites and even different domains and
forests, if that is how your Active Directory architecture is laid out (we??™ll get into the Active
Directory architecture later in this chapter). Replication is built into Active Directory
so that if you??™re working with multiple domain controllers, a change to any of them is
automatically replicated to the others and is governed by a set of synchronization rules.
Active Directory is extensible??”that is, the schema that defines how objects look in
Active Directory can be modified. For example, if you install Exchange 2003 or a later
version into an Active Directory domain, it will modify the schema so that a user object
not only contains the standard user information but also information Exchange might
want to use, such as the location of a mailbox and additional e-mail addresses tied to the
user.
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