Communication system, a communication method, a relay device, and a communication program

Communication system includes an information processing apparatus and the relay apparatus. Relay apparatus stores information for identifying the transfer destination device is a transfer destination of a packet containing the data destined for the information processing apparatus, and relays communications between the first network and a second network including a destination device . Relay apparatus includes a transmission unit and generator. Generating unit, from the second transmission source in the network, when receiving the first data packet containing data addressed to the information processing apparatus, when specifying the device relay apparatus includes a transmission source to the second network generating a device index values ​​to be used. Transmission unit, the second data packet added to the network index value identifying device index values ​​to the first data packet and the second network, it transmits to the destination device.