Not know what you are needing, this site may explain how ftp works. This was passed on to me by another user when I was having problems.
http://slacksite.com/other/ftp.html
Passive is not required behind a firewall. Port 21 is still good. You will find that setting up security can cause head achs. But must be correct for connections, mormal and ftp.
If you ned to test your connection from the outside world this site may help
http://www.g6ftpserver.com/en/ftptest
or have a friend test your connection.