The FTP uses two communication channels: control connection and data connection.

Control connection

• Control connection is established on a well-known port.
• Is used for sending commands to the FTP server.

Control connections use the following ports:

• port 21 - plain old FTP
• port 21 - FTP over TLS/SSL Explicit mode
• port 990 - FTP over TLS/SSL Implicit mode

Data connection

• Used for upload, download, and directory listing transfer.
• Initiated either by the FTP client (passive mode) or by the FTP server (active mode)
• Established on random port. In passive mode, the port is selected by the FTP server. In active mode, it is selected by the FTP client.

See the following links for more info:

• INFO: TLS/SSL implicit and explicit modes difference - includes list of well-known ports for control connections for both plain FTP and FTP/SSL.

• FTP Active and Passive mode difference - includes communication log of typical active and passive FTP mode session

• Questions tagged 'passive-mode' on this forum about passive/active FTP mode