I have a 100mb network. I want to put a second NIC in both the server and a workstation. These NICs would be 1000mb. Would I be able to have the existing 100mb NICs to receive internet from the router and to talk to all the other computers on the network, and the 1gb NICs just for the server & workstation to talk, for fast NFS access? I don't understand how the operating system(?) knows which NIC to use to talk to the server and back to the workstation. The server is OpenBSD and the workstation Debian unstable, if that matters. Thank you.

This question probably belongs in the networking forum, but I think I know how to do what you want. Make a subnet just for gigabit (if your normal net is 192.168, use 172.16, for example) by configuring the hostname.whatever file in OpenBSD with some IP and a netmask, and configure Debian with a different IP (on the same net, obviously) and the same netmask. The OSs should know which NIC to use by inspecting the IP, with no weird route commands or packet forwarding.

You could also modify your routing table to specify which network interface (eth0 or eth1) handles traffic to certain IPs. If you second server has an IP of 192.168.0.2 you can specify traffic going to that IP to use the second card with somethihng like: # route add -net 192.168.0.2 netmask 255.255.255.255 eth1

Thanks for the comments. 3Dave's method is the easiest and seems to work well being tested on 100mb cards. Thanks again.