Abstract: A communication system which includes first to third terminals which belong to a first network, and the first and the third terminals belong to a second network. The first terminal transmits a first packet generated by the first terminal to a second terminal through the first network. When the second terminal receives the first packet, the second terminal monitors a route between the first and the second terminals. When the monitored results satisfy prescribed conditions, the second terminal transmits a second packet to the third terminal. When the third terminal receives the second packet, the third terminal transmits a third packet, which indicates that the third terminal can be connected to the second network, to the second terminal.