MCSA/MCSE Self-Paced Training Kit (Exam 70-270): Installing ...

Lesson 2 Important Active Directory Concepts
14-15
Schema class objects describe the possible Active Directory objects that can be created.
Each schema class is a collection of schema attribute objects. For each object class, the
schema defines which attributes an instance of the class must have, which additional
attributes it can have, and which object class can be a parent of the current object class.
Every object in Active Directory is an instance of a schema class object.
Schema attribute objects define the schema class objects with which they are associated.
Each schema attribute is defined only once and can be used in multiple schema
classes. Because the schema definitions are themselves stored as objects in Active
Directory, they can be administered in the same manner as the rest of the objects in
Active Directory.
Installing Active Directory on the first domain controller in a network creates a default
schema that contains a set of basic schema class attributes. The default schema also
contains definitions of objects and properties that Active Directory uses internally to
function.
The Active Directory schema is extensible, which means that you can define new directory
object types and attributes and new attributes for existing objects. For example,
Microsoft Exchange Server extends the schema to add e-mail properties to user
account information in Active Directory. You can also extend the schema directly by
using the Schema Manager snap-in or the Active Directory Service Interface (ADSI).
Only experienced developers or network administrators should dynamically extend
the schema by defining new classes and attributes for existing classes.
The schema is implemented and stored within Active Directory itself (in the Global Catalog),
and it can be updated dynamically. As a result, an application can extend the
schema with new attributes and classes and use the extensions immediately.
Note Write access to the schema is limited to members of the Schema Admins group by
default.
What Is the Global Catalog?
Active Directory allows users and administrators to find objects (such as files, printers,
or users) in their own domain. However, finding objects outside of the domain and
across the enterprise requires a mechanism that allows the domains to act as one
entity. A catalog service contains selected information about every object in all
domains in the directory, which is useful for performing searches across an enterprise.
The catalog service provided by Active Directory services is called the Global Catalog.
The Global Catalog is the central repository of information about objects in a tree or
forest, as shown in Figure 14-7. By default, a Global Catalog is created automatically on