I am glad to see the latest version of ktorrent has support for binding to a specific IP address. This is great news for those of us with multiple NICs and/or multiple ISPs.

But this might not work as you expect, without fiddling with the routing tables a bit. The goal of this feature for most people is to direct all torrent traffic through one network interface. The problem is, a socket bind on outgoing traffic only specifies the source address of the IP packet, NOT the interface used by the host to send the packet. The host normally routes the packet by destination, not by source. You can still have the host deciding to send the packet by the wrong interface, even if you bind the socket to the right IP address!

How do you solve this? By using source-based routing, also called policy-based routing. This is actually quite easy to do. Here is a web page which describes what you need to do and why:

http://www.linuxjournal.com/article/7291

This article talks about a single ISP, but it is easily adapted to multiple ISP (e.g. if you have each NIC connected to a different ISP). You simply make the two tables ("tab 1" and "tab 2") have a different gateway IP address corresponding to the ISP that each NIC is attached to.

BTW, this is something that linux boxes can do, but windows boxes can't.