With a little quick soldering, you can convert your Linksys router to receive it’s DC power through the ethernet uplink instead of through a separate power cable. This is incredibly useful if you want to place your router above the ceiling or in a location that is inconvenient to get power to.
After the conversion, the 12v DC from the power adapter is sent along the unused blue and brown ethernet pairs. You’ll be able to place your router over 500ft from the switch without the voltage dropping below the 5.5v that the router needs to operate.
HOWTO: WRT54GL DIY Internal POE! (Google cache) – Link