Use either the active or passive mode to connect to a File Transfer Protocol (FTP) server.
Active mode vs. passive mode
FTP utilizes two ports, a data port and a command port, to transfer information from a client to a server. Typically, the command port uses port 21 and the data port uses port 20. When you use a different mode, however, the data port does not always use port 20.
In active mode, the FTP server responds to the connection attempt and returns a connection request from a different port to the FTP client. Network Address Translation (NAT) configurations block this connection request.
In passive mode, the FTP client initiates both connection attempts. NAT configurations do not block this connection request.
If FTP users exist on the private network side of a NAT configuration, you must enable FTP’s passive mode and open the passive port range in your FTP server’s configuration file. You may also need to open the passive port range on your firewall.