In the FTP protocol two ports are normally used, 20 and 21.

Port 21 is the control channel in which requests such as PUT, GET, and PWD are sent to the server.

Port 20 is the data channel in which actual files, folders and directory listings are sent.

Normally, when you connect to an FTP site, the site establishes the data connection to your PC (the client) on Port 20. However, if the site allows passive transfers, you can have your PC establish the data connection. We recommend that you use passive mode for most transfers.

Some ftp servers support both active and passive transfers while others only allow active transfers.

In some cases users who are behind some types of router-based firewalls or behind a gateway requiring passive transfers will require passive mode.

You will typically get a "Blocking Call Cancelled" error message if the transfer method you are using is rejected by the server.