Abstract: A method is provided for operating a field bus network system having a ring topology, which has a master unit and a plurality of slave units, which are connected by a field bus in a ring topology.

Abstract: An address allocation system including a client and a server is provided. The server includes a setting module, a detecting module, and a converting module. The setting module is for setting a filtering code. The detecting module is for detecting whether a physical-layer address of the client corresponds to the filtering code. The converting module is for converting the physical-layer address into a network-layer address if the physical-layer address corresponds to the filtering code, and for allocating the network-layer address to the client. An address allocation method is also provided.