The general syntax for entering port numbers are [IP:]<Port>[,]... Please observe that the usage of an IP is totally optional, and if omitted it will be treated as "0.0.0.0" (i.e. all available IPs).

Listen on the same port for all available IPs: The FTP server will be active on all available IPs on the computer and listen on port 21.

Listen on several ports for all available IPs: The FTP server will be active on all available IPs on the computer and listen on ports 21, 900, and 3500.

Listen on several ports, on specific IPs: The FTP server will listen on all available IPs on port 6021, and on IP 66.94.234.20 on port 21.