Windows 2000 DCs use Lightweight Directory Access Protocol ( LDAP) port 389 to communicate with the AD. Port 389 is used for client queries and administrative tasks. If your Windows 2000 DC is running Exchange Server, you must make some changes, as Exchange Server uses LDAP port 389.

If you do NOT make changes, there WILL BE mis-communication between data directed to the AD and data directed to the Exchange Directory store. Your Application event log may also contain

Event ID: 1306                              Type: Error                              Source: MSExchangeDS                               Description: Register LDAP protocol failed with error 10048. If port number 389                              is used by another application, change to an unused port, then shut down and                               restart Microsoft Exchange Directory Services.                               Contact Microsoft Support Service if condition persists.                               Event ID: 1309                              Type: Error                              Source: MSExchangeDS                              Description: Register LDAP SSL protocol failed with error 10048. The LDAP SSL server                              is not available. Make sure port number 636 is not used by another application.
Both products also use LDAP port 636 for secure data transmission, so this must also be changed.

You can modify Exchange LDAP to listen on different ports, so that 389 and 636 are available for AD.

See Microsoft Knowledge Base article Q224447 - How To Change LDAP Port Assignments in Exchange Server.

NOTE: Exchange does NOT have to be modified for:

LDAP port for Global Catalog: 3268
LDAP port for Global Catalog with SSL: 3269.