By checking the Use custom port range checkbox you can set up a range of ports that FileZilla Server will use for passive mode. The default custom range goes from 49152 to 65534. The total number of concurrent data connections is limited by the size of the range. Consider setting the range greater than the number of transfers that will take place in a 4 minutes interval.
If you need the FileZilla Server to be reachable via the internet via its own IP address, enter the host in the Use the following host field. The host can be either an external IP address (for instance, the public IP address of the NAT router) or a hostname that resolves to such IP address.
By checking the Use default host for local connections checkbox FileZilla Server manages passive data connections through the local IP address.