Abstract: A network system has a plurality of networks. A first internetwork apparatus has a plurality of first ports each connected to the plurality of networks and a second internetwork apparatus has a plurality of second ports each connected to the plurality of networks. A data transmission path is connected to the first and second internetwork apparatuses to transmit data therebetween. In the normal state, each of the plurality of first ports is able to transmit and receive data to and from one of the plurality of networks while the plurality of second ports are able to transmit, but not receive data from the plurality of networks. A failure occurring in one of the plurality of first ports and a route therefrom to one of the plurality of networks is detected.