To use this page, you must select a dynamic DNS service, enter the hostname used by your system on that service, and also the service's login details. By default, Virtualmin will look at the IP address assigned to the primary network interface (like eth0) to check for changes, but if your system is behind a NAT router you should select the Address visible from the Internet option to have it discover the IP that the rest of the Internet really sees.
When a change is detected, all virtual servers with shared IP addresses using the old address will be updated to the new one. This change will be made in both the Apache configuration and DNS records.