I was trying to install the FreeBSD 7.0 in my VMWare virtual machine (VM). After I downloaded the installer disk's ISO image, I connected that ISO file as my CDROM drive in my VM and booted my VM. I should mention that my VM is in NAT mode, as I had decided to install the FreeBSD over the network.
When the installation tries to download the distro files from the FTP servers, it just kept saying it could not connect to the FTP server. I wasted around an hour or so without thinking about the basics (Damn!) that I am behind a NAT and I am trying to download files using active FTP mode. Then I tried to connect in passive mode and everything worked fine.
I don't want to repeat what many people have already clearly explained about passive mode of FTP. Here is an excellent tutorial that talks about active and passive modes of FTP.
Moral of the story is, don't forget that active FTP doesn't work (generally :-) if you are behind a NAT.
I would like to add only one thing: I think it would be a nice idea to have the FreeBSD installer indicate that passive ftp mode could work, in case it fails to open the data connection in active mode.