Communication system, server, router, and mobile communications terminal

A communication system including a plurality of packet routers and a

• server connected over a network to first and second terminals, wherein
• the server includes a memory for storing the match between the address in the home network and the address of the movement destination of the terminal managed by the server itself, and
• at least one packet router among the plurality of packet routers includes:
• a transmit section for sending to a first terminal, information on a network where at least one other router among the plurality of packet routers is connected, and information on the network where its own packet router is connected, and
• the first terminal includes a transmit section to send two care-of addresses generated respectively from information on a network connected to the first packet router and information on a network connected to another packet router, and
• the server or the second terminal that has received the two care-of addresses includes:
• a transmit section to send packets with duplicate contents to the two care-of addresses that have been received.