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#21
Normally, a house or apartment that is wired for ethernet will have jacks in each room and then in a closet or utility room there will be an area to plug in a router, modem, or switch.
in that room will either be loose wires with jacks on them or a plate with however many jacks that are wired into the house.
Basically, each of the rooms has an open circuit at the ethernet port if the device in the closet/utility room is missing.