A packet sent in a mobile IP communication between different regions passes through a particular address translation unit, so that the communication route cannot be optimized. The packet cannot be delivered to a mobile node not having a fixed home address. To overcome the difficulties, the address translation unit includes a function to process the mobile IPv6 protocol. The mobile node includes a function to dynamically acquire a home address. A first network and a second network each identify the mobile node by a SIP (session initiation protocol) identifier. The mobile node acquires a home address on the second network and registers a corresponding relation between the identifiers and the home address thereof. A terminal on the second network acquires the home address of the mobile node from information of the mobile node registered to the SIP server according to a SIP procedure.