However, it also means that you need to open at least two ports on the server side: the FTP command port (21) and one or more FTP data ports (which can be anything above 1024) and then tell the server to use those ports for data. This means that the client doesn't need to mess around with incoming firewall rules or NAT port forwarding. Passive FTP has the client initiate both connections. To avoid this problem, you need to use Passive FTP. However, if your client is behind NAT (that is different from your server's NAT) then the client will not be able to use Active FTP because the port that the client needs to listen on will not be forwarded from the NAT router.
A service like CanYouSeeMe is probably configured to support this, automatically opening the incoming port when needed. Active FTP requires that the client be able to listen on an incoming port.
