How does Active FTP work?

In active mode FTP the client connects from a random unprivileged port (port number > 1023) to the FTP server's command port, port 21. Then, the client starts listening to port N+1 and sends the FTP command PORT N+1 to the FTP server. The server will then connect back to the client's specified data port from its local data port, which is port 20.

Firewall on Active FTP - what ports should be opened on server and client side?

On server port 20 and port 21 must be opened on the client (home user) ports higher than 1023 must be opened for successfull ftp connection and transfer of files.