Are you sure you are asking the right question? There are lots of Ethernet standards, I assume your are talking about 100BASE-TX?
You can not directly put a DC voltage on an Ethernet connection using standard hardware (beyond other problems it is physically impossible because it is transformer-coupled on both sides). You can transmit a message (frame), which is a one-time thing. Or you can transmit repeatedly, but that still won't get you a DC signal. And if you transmit nothing, that will not guarantee that there is no signal on the Ethernet connection!
In short, you won't be able to do much with your Ethernet connection without an intelligence (microcontroller) on the oher side.