Reference: FTP Server Administration options
This field specifies the IP address that is published by the local server when issuing responses to the PASV command. It is particularly useful when employing SSL behind a firewall that performs Network Address Translation (NAT). Under clear channel conditions, the firewall inspects FTP packets and makes adjustments in the PASV and PORT information that is exchanged between a client and the server. Under encrypted conditions, this inspection is impossible. You must provide an address that is addressable on the Internet (usually the address of a front line router), and you must provide a port range that is open through your firewalls.