a computer readable medium storing a computer program for causing a computer of a mobile terminal device capable of accessing both a mobile communication network and a local network simultaneously , to execute an address assignment , the computer readable medium including first computer program codes for causing the computer to transmit an inquiry message for inquiring information regarding a location on networks of a correspondent of packet communications to be carried out , from the mobile terminal device through the local network , second computer program codes for causing the computer to receive a response message in response to the inquiry message through the local network , third computer program codes for causing the computer to acquire an address to be used by the mobile terminal device in the packet communications with the correspondent according to the response message , fourth computer program codes for causing the computer to assign the address to a first communication interface for the local network at the mobile terminal device , and fifth computer program codes for causing the computer to judge a manner of utilizing first packet communications carried out through the first communication interface and second packet communications carried out through a second communication interface provided for the mobile communication network in the mobile terminal device according to a prescribed policy , when both the first packet communications and the second packet communication are possible .