disabling ethernet at the motherboard level will not disable the ethernet adapter in your cpu/gpus. Thus disabling it for the rest of your computers would allow network attacks via those adapters on those machines to use the same wire to sniff traffic on internet-facing ports or access data from other hosts on the network connected to that same cable because the attack will be over the network interface card on your cpus instead of using the card itself like it would if the interface were disabled on the motherboard level... those servers are not online but are physically disconnected through the power off switch so that means that there will be no network traffic on that port anymore thus since there are no switches between the server and the routers meaning no packets to be picked up it will be the same as if you had de-enabled the interface by going into the bios and selecting to disable the network as well since you no longer have any interfaces you system will not have a way of sending and receiving ip packets from your network to your modem therefore when the wan connection is active you will not be able to send or receive packages back and fourth between your computer or anyother computer's from the outside world this is how you will stop unwanted traffic flowing from your computer by using a simple power-off switch this will stop you from having someone sniffing all your network and internet traffic when your not around to stop them.. you can block such access to your computer easily using windows xp firewall controls by just selecting the option to block all incoming requests which will take care of the problem.... hope this helps you understand a bit more about how the lan communication in windows