To ensure successful data migration, Data Transmission Service (DTS) checks whether DTS servers can connect to the source and destination databases during precheck.
If the Access Method parameter is set to Cloud Enterprise Network (CEN) or Express Connect, VPN Gateway, or Smart Access Gateway, DTS checks whether the CIDR blocks of DTS servers are added to the security settings of the source and destination databases during precheck. If the precheck fails, you can troubleshoot the issue based on the message in the CIDR Block of Source Database or CIDR Block of Destination Database column on the Connectivity Check for Express Connect tab. For more information about how to troubleshoot the issue, see What do I do if an error is reported when I connect a database instance to DTS over VPN? or What do I do if an error is reported when I connect a database instance to DTS over CEN? For more information about the CIDR blocks of DTS servers, see Add the CIDR blocks of DTS servers to the security settings of on-premises databases.