When synchronizing data between two Active Directory domains, Synchronization Service uses the following ports to access domain controllers in the domains:
| Port | Protocol | Type of traffic | Direction of traffic | 
| 53 | TCP/UDP | DNS | Inbound | 
| 88 | TCP/UDP | Kerberos | Outbound | 
| 389 | TCP/UDP | LDAP | Outbound | 
| 636 | TCP | LDAP over SSL (LDAPS) | Outbound | 
