In passive mode, the server does not receive an IP address from the client (due to a firewall, for example) and offers the client a port through which a connection can be established. This process informs the server which port the client can be reached on.
In active mode, the client, which uses port 1023, signals its IP address through port 21 during connection buildup. At this point, it’s important to differentiate between two different types of transfer modes. Following this, the data is transferred through another port. This FTP client creates a TCP connection to the control port of the server (normally port 21) and is then able to send commands that the server subsequently answers. In order to reach an FTP server, a connection through an FTP client first needs to be established.
