If your Virtualmin server is on an internal private network,
and all HTTP traffic is forwarded to it via a firewall or proxy, you should
set this field to the IP address via which it is accessed externally, such
as that of your firewall or router. When this option is set, the address
entered will be used for all DNS records that would normally contain the real
address, such as www, mail and ftp.
By default, the DNS records automatically generated for this virtual server
will contain the IP address that it is assigned by Virtualmin. However, in
some cases this address may not actually be the one you want web and mail
clients on the Internet to connect to.