A communication system which includes plural radio systems, and which connects to a terminal apparatus corresponding to the plural radio systems, comprising: access points for the each radio system; gateways; an authentication apparatus; and a home agent. The authentication apparatus being shared by the plural radio systems: assigns an IP address to the terminal apparatus in a radio system which is initially connected among the plural radio systems according to an IP address assignment request from the terminal; judges whether an IP address assignment request for a radio system to be subsequently connected is from the same terminal apparatus by using an unique identifier of the terminal; and assigns the IP address assigned in the initially connected radio system to the terminal apparatus in the radio system to be subsequently connected in a case where the assignment request is determined to be from the same terminal apparatus.