The schema contains the definition for all the objects (example: user, computer and printers) in the directory. If we create an object in an active directory, it is validated against the corresponding object definition in the schema before being record to the directory. The schema is made up of classes and attributes. The default schema contains a set of object classes and attributes to meet our desired needs. Schema is extensible; we can modify and add attributes and classes to the base schema. Schema changes can affect the entire network. So before changing the schema, test in the lab environment.