If you're looking to connect a DC-powered device to a standards-based power-over-Ethernet switch without making any device modifications look up "PoE splitters"

If you're looking to connect a DC-powered device to a standards-based power-over-Ethernet switch without making any device modifications look up "PoE splitters". They're really, really handy devices. You can get them with USB connections and barrel plugs. I've gotten a lot of mileage out of them over the years.