In order to timely and accurately estimate the number of occupants in a room, and then to understand the occupancy distribution for various areas in a building, an addressable active infrared intrusion detector using an IP address as its identification was designed in this paper. Differently from the traditional infrared detectors that were always attached to the host, the detector was achieved to communicate with the host computer through TCP sockets, rendering one host could receive messages from multiple detectors through network configurations. Our designed addressable active infrared intrusion detector composed of active infrared intrusion detector, microprocessor and wireless communication module. The active infrared intrusion detector collected the data during the occupants’ entering and exiting process; the microprocessor processed the perceived signals; the data was transmitted to the host computer by the wireless communication module. Furthermore, based on the detector, an occupant counting program accounting for the entering and exiting was developed. Experiments results shown that the addressable active infrared intrusion detector could accurately and timely detect changes in occupants and transmitted messages reliably to the host computer, and the occupant counting program could accurately estimate the number of occupants in a given area.