How FTP Works

FTP works by opening two different connections between your FTP program (also called an FTP Client) and the FTP Server (FTP site you are trying to connect to). These connections are opened on what is called ports. A port is like a passageway for data to flow between two computers. The first connection opened is on Port 21, which is the standard FTP port and is the Command connection. This is the port where commands are send from your computer to the FTP server to tell it what you want to do, be it listing a directory, moving to another one, deleting a file, uploading or downloading a file, or changing file permissions, etc. You probably have seen these commands pop up in the message area of your FTP program, like RETR, CWD, and LIST for example. The second connection is where the actual data (like a file) is transferred between your computer and the server. Now the port that the transfer takes place on depends on whether you have your FTP program set to Passive mode or Active mode.

Passive (PASV)

When your FTP program is in Passive mode, it opens a data communications port between your computer and the server on port 21 by using a PASV command. Then the server will respond back telling your FTP program what IP address and port to connect to for the actual transfer of data. Your FTP program will then connect to that IP address and port, and the transfer of the file will start. If you are using a corporate firewall or even a personal firewall such as ZoneAlarm or the firewall in Windows XP, these should be able to let you use Passive mode. This is because most firewalls will see this data transmission as outbound traffic and normally should not attempt to block it. If for some reason you are having problems with Passive mode, such as the directory listing hanging or stalling during data transfers, you can always try Active mode.

Active (PORT)

Active mode works the opposite direction of Passive mode in that instead of the server determining the IP address and port to connect to, your FTP program will give this information to the FTP server. So when data transmission is requested, your FTP program will issue a PORT command and the IP address and port number it wants to use for the data connection. The FTP server will then connect directly to your FTP program on that IP address and port and data transfer will start. If the FTP server is behind a firewall (as is Active Web Hosting's FTP server), then this is the mode you should use.

Which mode should I use?

Because Active Web Hosting's server is behind a firewall, it's recommended that you have your FTP program set to Active mode. However, if you are having problems with this connection, you may try Passive mode. You might see messages in your FTP client from the server recommending "PASV" or Passive mode. You shouldn't need to change to this mode though unless you are experiencing problems.