GCs. SMTP cannot be used for replicating the default Domain partition. Implementation In general, a network utilizing Active Directory has more than one licensed Windows server computer. Backup and restore of Active Directory is possible for a network with a single domain controller, but Microsoft recommends more than one domain controller to provide automatic failover protection of the directory. Domain controllers are also ideally single-purpose for directory operations only, and should not run any other software or role. Certain Microsoft products such as SQL Server and Exchange can interfere with the operation of a domain controller, necessitating isolation of these products on additional Windows servers. Combining them can make configuration or troubleshooting of either the domain controller or the other installed software more difficult. A business intending to implement Active Directory is therefore recommended to purchase a number of Windows server licenses, to provide for at least two separate domain controllers, and optionally, additional domain controllers for performance or redundancy, a separate file server, a separate Exchange server, a separate SQL Server, and so forth to support the various server roles. Physical hardware costs for the many separate servers can be reduced through the use of virtualization, although for proper failover protection, Microsoft recommends not running multiple virtualized domain controllers on the same physical hardware. Database The Active-Directory database, the directory store, in Windows 2000 Server uses the JET Blue-based Extensible Storage Engine (ESE98) and is limited to 16 terabytes and 2 billion objects (but only 1 billion security principals) in each domain controller's database. Microsoft has created NTDS databases with more than 2 billion objects. (NT4's Security Account Manager could support no more than 40,000 objects). Called NTDS.DIT, it has two main tables: the data table and the link table. Windows Server 2003 added a third main table for security descriptor single instancing. Programs may access the features of Active Directory via the COM interfaces provided by Active Directory Service Interfaces. Trusting To allow users in one domain to access resources in another, Active Directory uses trusts. Trusts inside a forest are automatically created when domains are created. The forest sets the default boundaries of trust, and implicit, transitive trust is automatic for all domains within a forest. Terminology One-way trust One domain allows access to users on another domain, but the other domain does not allow access to users on the first domain. Two-way trust Two domains allow