1. A method of operating a station to selectively access a communications medium that is divided into time slots by demarcation signals transmitted from a controller at regular intervals, the method comprising: receiving a demarcation signal that demarcates a respective one of the time slots and that contains one or more interrogatory codes and a control mask; masking each of the one or more interrogatory codes from the received demarcation signal with the control mask to produce, for each interrogatory code, a respective masked interrogatory code; masking a response code stored in the station with the control mask to produce a masked response code; barring an attempt to access the communications medium in the respective time slot demarcated by the received demarcation signal if none of the one or more masked interrogatory codes matches the masked response code; joining a network maintained by the controller; and receiving, from the controller, a control signal containing a new response code.