mnesting
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By default, CuteFTP will automatically make connections in PASV mode. If the PASV connection fails, CuteFTP automatically switches to PORT mode and attempts the connection. Sometimes, it becomes necessary to switch modes manually. To make the switch for a particular site, follow these steps:
CuteFTP Pro: In the Site Manager, right-click on the problem site and choose Site Properties, click the Type tab and change the Data Connection Type to the desired mode.
CuteFTP Home: In the Site Manager, click on the problem site and then on the right side, click the Type tab and change the Data Connection Type to the desired mode.
Here's a brief explanation about PORT and PASV data connection modes:
FTP is a protocol that uses two separate channels: the command channel and the data channel. The command channel is used for the initial connection, login, sending commands and receiving server responses, etc. By default the command channel is established on port 21, however the data channel must also be established for any data transfer to occur at all, whether it's uploading a file or simply getting a directory listing from the server. There are two ways to establish the data channel.
PORT (Active) Mode: CuteFTP takes a very aggressive, active role in establishing the data channel connection. Over the command channel via port 21 it squares its shoulders, looks the remote server in the eye, and tells the server, "Connect back to me on my IP address and my port!" The remote server obeys and makes an outbound connection to CuteFTP at the IP address and port specified by CuteFTP. Once that data channel is established, the data transfer takes place.
PASV (Passive) Mode: CuteFTP takes a very passive role in establishing the data channel connection. It meekly sends four letters: PASV. It's basically lowering its eyes and saying, "Server, you tell me what you want me to do, and I'll obey." So the remote server replies, "OK, cool. You connect to me at my address and my port." CuteFTP then dutifully obeys and creates a new outbound connection to the server at the IP address and random port specified by the remote server. Once that data channel is established, the data transfer takes place. PASV mode is necessary for some firewalls and routers, because with PASV CuteFTP opens the connection to an IP Address and port that the server supplies.
Thanks to Deacon for his help in providing most of the explanation above.
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PORT vs. PASV (Active vs. Passive) Data Mode - 
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