btw, does this line is really needed? (lots of people are being stuck by this parameter)
Read the doc or the web page for a description of what it does, and you won't have any problems. :-)
Short explanation: In response to the PASV command, the FTP server on the DS replies with the IP address and port that its data connection is at. If you're behind a NAT router that forwards FTP to your DS, the DS' IP address will not be visible and the client won't be able to connect.
In that case, you need to put the router's hostname/IP there. That's why the doc talks about masquerading.
If you're only on your local net, this is not needed at all.