Maximum Number of Objects
Each domain controller in an Active Directory forest can create a little bit less than 2.15 billion objects during its lifetime.
Each Active Directory domain controller has a unique identifier that is specific to the individual domain controller. These identifiers, which are called Distinguished Name Tags (DNTs), are not replicated or otherwise visible to other domain controllers. The range of values for DNTs is from 0 through 2,147,483,393 (231 minus 255). As objects are created on a domain controller, a unique value is used. A DNT is not reused when an object is deleted. Therefore, domain controllers are limited to creating approximately 2 billion objects (including objects that are created through replication). This limit applies to the aggregate of all objects from all partitions (domain NC, configuration, schema, and any application directory partitions) that are hosted on the domain controller.
Because new domain controllers start with low initial DNT values (typically, anywhere from 100 up to 2,000), it may be possible to work around the domain controller lifetime creation limit—assuming, of course, that the domain is currently maintaining less than 2 billion objects. For example, if the lifetime creation limit is reached because approximately 2 billion objects are created, but 500 million objects are removed from the domain (for example, deleted and then permanently removed from the database through the garbage collection process), installing a new domain controller and allowing it to replicate the remaining objects from the existing domain controllers is a potential workaround. However, it is important that the new domain controller receives the objects through replication and that such domain controllers not be promoted with the Install from Media (IFM) option. Domain controllers that are installed with IFM inherit the DNT values from the domain controller that was used to create the IFM backup.
At the database level, the error that occurs when the DNT limit is reached is “Error: Add: Operations Error. <1> Server error: 000020EF: SvcErr: DSID-0208044C, problem 5012 (DIR_ERROR), data -1076.”
Maximum Number of Security Identifiers
There is a limit of approximately 1 billion security identifiers (SIDs) over the life of a domain. This limit is due to the size of the global relative identifier (RID) pool of 30 bits that makes each SID (that is assigned to user, group, and computer accounts) in a domain unique. The actual limit is 230 or 1,073,741,824 RIDs. Because RIDs are not reused—even if security principals are deleted—the maximum limit applies, even if there are less than 1 billion security principals in the domain.
When all the available RIDs are assigned for a domain, the Directory Service log in the Application and Service Logs of Event Viewer also displays Event ID 16644 from an event log source of the Security Accounts Manager (SAM) that reads “The maximum domain account identifier value has been reached. No further account-identifier pools can be allocated to domain controllers in this domain.”
A partial work-around to this limitation is to create an additional domain to hold accounts and then migrate accounts to the new domain. However, you must create a trust relationship to migrate accounts in advance of reaching the limit. Creating a trust requires the creation of a security principal, which is also known as a trust user account. For more information about this limit, see articles 316201 (http://go.microsoft.com/fwlink/?LinkID=115211 ) and 305475 (http://go.microsoft.com/fwlink/?LinkId=115212 ) in the Microsoft Knowledge Base.
Group Memberships for Security Principals
Security principals (that is, user, group, and computer accounts) can be members of a maximum of approximately 1,015 groups. This limitation is due to the size limit for the access token that is created for each security principal. For more information, see article 328889 in the Microsoft Knowledge Base (http://go.microsoft.com/fwlink/?LinkID=115213 ).
FQDN Length Limitations
Fully qualified domain names (FQDNs) in Active Directory cannot exceed 64 characters in total length, including hyphens and periods (.). As an example, the following host name has 65 characters and therefore is not valid in an Active Directory domain: server10.branch-15.southaz.westernregion.northamerica.contoso.com. This is an important limitation to keep in mind when you name domains. For more information about naming limitations, see article 909264 in the Microsoft Knowledge Base (http://support.microsoft.com/kb/909264 ).
File Name Length Limitations
The file system that Windows operating systems uses limits file name lengths (including the path to the file name) to 260 characters. That limitation applies also to physical files that Active Directory components use, such as SYSVOL and database file paths. When you are determining where to place your SYSVOL and database files during Active Directory installation, avoid nested folder structures that might make the full file path to the SYSVOL folder longer than 260 characters. For more information, see article 245809 in the Microsoft Knowledge Base (http://go.microsoft.com/fwlink/?LinkId=115219 ).
Organizational Unit Name Length
The maximum length for the name of an organizational unit (OU) is 64 characters. This limitation prevents an OU name from surpassing the file system limit of 260 characters when Universal Naming Convention (UNC) paths are constructed for Group Policy. For more information, see article 245809 in the Microsoft Knowledge Base (http://go.microsoft.com/fwlink/?LinkID=115219 ).
Maximum Number of Group Policy Objects Applied
There is a limit of 999 Group Policy objects (GPOs) that you can apply to a user account or computer account. This does not mean that the total number of policy settings on the system is limited to 999. Rather, a single user or computer will not be able to process more than 999 GPOs. This limit exists for performance reasons.
Maximum Number of Accounts per LDAP Transaction
When you write scripts or applications that perform Lightweight Directory Access Protocol (LDAP) transactions, the recommended limit is to perform no more than 5,000 operations per LDAP transaction. An LDAP transaction is a group of directory operations (such as add, delete, and modify) that are treated as one unit. If your script or application performs more than 5,000 operations in a single LDAP transaction, you are at risk of running into resource limits and an operational timeout. If that happens, all the operations (changes, additions, and modifications) in the transaction are rolled back, which means that you lose all those changes.
For example, if you are using Active Directory Service Interfaces (ADSI) to write a script, the SetInfo method completes a transaction. For more information about ADSI Methods, see Active Directory Service Interfaces (http://go.microsoft.com/fwlink/?LinkID=4487 ).
As another example, when you use the System.DirectoryServices (S.DS) namespace in the Microsoft .Net Framework, the DirectoryEntry.CommitChanges method completes an LDAP transaction. For more information about the DirectoryEntry.CommitChanges method, see DirectoryEntry.CommitChanges () (http://go.microsoft.com/fwlink/?LinkId=115220 ).
Recommended Maximum Number of Domains in a Forest
For Windows 2000 Server, the recommended maximum number of domains in a forest is 800. For Windows Server 2003, the recommended maximum number of domains when the forest functional level is set to Windows Server 2003 (also known as forest functional level 2) is 1,200. This restriction is a limitation of multivalued, nonlinked attributes in Windows Server 2003.
Recommended Maximum Number of Domain Controllers in a Domain
Because the File Replication Service (FRS) is used to replicate SYSVOL in a Windows Server 2003 domain, we recommend a limit of 1,200 domain controllers per domain to ensure reliable recovery of SYSVOL.
If any Active Directory domain in your network is expected to exceed 800 domain controllers and those domain controllers are hosting Active Directory–integrated Domain Name System (DNS) zones, review article 267855 in the Microsoft Knowledge Base (http://go.microsoft.com/fwlink/?LinkId=115222 ).
For more information about FRS limitations, see the FRS Technical Reference (http://go.microsoft.com/fwlink/?LinkId=115302 ).