1. A secure entry system for a building having a plurality of units provided therein, each of said units permitting only entry of a designated person thereto, said system comprising: an administration server, which is provided in said building, operable to administrate a computer network system in said building; and at least one interconnecting device operable to connect said administration server and respective network devices in said plurality of units, wherein said administration server sets said interconnecting device to assign a plurality of VLANs to each of said units, respectively, in such a manner that a VLAN assigned to one of said units is different from VLANs assigned to respective others of said units to provide secure entry to said units and said computer network system in the building.