BT FAQ's Natcheck The definitive way to check whether you are set up to allow inbound connections (through your firewalls). http://www.btfaq.com/natcheck.pl
Multiple computers on one router? You need static IP addresses When there's more than one computer connected to one router, you pretty much always need to set static IP address(es) for the computer(s) you want to use for BitTorrent. Otherwise, the router will assign different addresses to a computer at different times, so the inbound connections will get forwarded to the wrong computer. Also note, YOU NEED TO FORWARD DIFFERENT PORTS TO THE DIFFERENT COMPUTERS, so that the connections meant for that computer get routed there. (Or in other words, you can't forward the same port to multiple computers at the same time!) Set a static IP for each computer running BT; set your router to forward inbound connections for a port (or a range of ports) to the static IP; and be sure your BitTorrent software is set to use the port(s). So, for example computer A might be assigned IP 192.168.1.10, and connections to port 7811 might be forwarded there. Computer B might be assigned IP 192.168.1.11, and connections to port 8811 might be forwarded to that IP address. Then, be sure the software running on each computer is set to use the right port(s). http://portforward.com/networking/staticip.htm
Why is being firewalled so bad? The point of the illustration here is - the red, firewalled users can't connect to each other at all. That may be most or all of the peers in a swarm! That page needs an illustration that shows how it looks with just 1, or 0, blue seeds - which happens on many torrents. If you're a firewalled peer on a torrent like that, you will have a lot of trouble uploading or downloading. If, on the other hand, you get set up right - you'll be very busy and in demand! http://bt.degreez.net/firewalled.html
dc FAQ at Broadband Reports If you don't find the info on your router or firewall at portforward.com (link above), check here. DC is similar to BT, in that people upload and download, with both inbound and outbound connections - but it uses different ports. So you'll have to configure using the BT ports. Try following the instructions for your router or firewall, but instead of using the DC port numbers, use the BT ports (usually 6881-6889 - though for many people, all you really need is 6881, at least for starters). http://www.dslreports.com/faq/dc