We do this all the time with devices carrying the DMX protocol. As others have said, it works only in networks that aren't using all 4 pairs of ethernet cable -

We do this all the time with devices carrying the DMX protocol. As others have said, it works only in networks that aren't using all 4 pairs of ethernet cable -- which is the case in DMX. The biggest issue when doing this is always sizing the power supply to the receiving device.