of the destination table 17 of the node control module 10 shown in fig5 showing correspondence between the destination ip address of the ip packet 902 and the internal address the connection control unit 418 controls packet reception from the extension module 51 the module authentication control unit 417 authenticates the extension module 51 the memory 420 stores an authentication condition table or the like described later by referring to fig8 the connector 419 is connected to a connector 519 of the extension module 51 the module connection sensor 416 determines whether the extension module 51 is connected or not to the connector 419 and informs a result of the determination to the module authentication control unit 417 for example the module connector sensor 416 is a pressure sensor an electric sensor or the like and only needs to detect connection of the extension module 51 to the connector 419 the extension module 51 includes an input unit 511 a display unit 512 a module control unit 513 a memory 514 a packet data statistics unit 515 and a connector 519 the input unit 511 receives information from the administrator and transmits the input information to the module control unit 513 the display unit 512 displays the information received from the module control unit 513 the input and display units 511 and 512 may be interfaces connected to the computer for inputting information to the module control unit 513 the module control unit 513 controls the entire extension module 51 the memory 514 stores an authentication.