Communication system and session establishment method

A communication system capable of securely preventing illegal use of addresses and enhancing confidentiality of communication. Based on an originating-side real address which is the real address of an originating-side client terminal and specific information included in a registration message, a conversion data generator generates conversion data which the local server alone can convert to a receiving-side real address which is the real address of a receiving-side client terminal, and transmits the conversion data to the originating-side client terminal. A session controller receives a first session establishment request message, obtains the receiving-side real address from the conversion data and the originating-side real address included in the first session establishment request message, and transmits a second session establishment request message to the receiving-side real address, whereby a session is established between the originating-side and receiving-side client terminals without directly using the originating-side and receiving-side real addresses uniquely assigned on the network.