TOPIC: Firewalls and "passive mode"...

If a computer is behind a firewall, usually it has problems in peer2peer connection programs, such as DC++. It's not possible to use "active mode", you have to use "passive mode". But two "passive mode" computers can't make a connection between them. Why?
I need to transfer files between two computers which are behind firewalls. If they both can access the same server, for example, www.firewall.cx, why can't they access each other? What could be the reasons?

Enable Server Services and make sure File Sharing is ON. You can map a network drive or go to the run box and type
\\IP\sharename. Type nbtstat -n which gives local name table and if you see <computer-name> and a hex value of <20>, you know File Sharing is enabled.

Thanks. I will keep your answer in mind. But I am afraid that the computer that I want to access has also a shared IP. But it's another question on another topic.