FTP can be run in active or passive mode, which determines how the data connection is established. If you're having trouble establishing an FTP connection to a server, using "passive" mode instead may help.
In active mode, the client sends the server the IP address and port number on which the client will listen, and the server initiates the TCP connection.
In situations where the client is behind a firewall and unable to accept incoming TCP connections, passive mode may be used. In this mode the client sends a PASV command to the server and receives an IP address and port number in return. The client uses these to open the data connection to the server.
Those hungry for even more information about FTP, and its intriguing passive mode, can turn to Wikipedia.
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