A communication system including a plurality of terminals (102∼107); and a switchboard connected to the terminals (102∼107), comprising: means for causing each of the terminals (102∼107) to register a user identifier, one or more of possible communication methods of the terminal (102∼107) itself, an address of the terminal (102∼107) itself corresponding to the possible communication methods, and one or more of port numbers corresponding respectively to the possible communication methods in the switchboard; means for, when one of the terminals (102∼107) transmits a request of transmitting a list of user identifiers connected to the switchboard, causing the switchboard to transmit the list to the terminal (102∼107) that transmits the request; means for, when the terminal (102∼107) that transmits the request selects one of the user identifiers from the list and transmits the selected user identifier to the switchboard, causing the switchboard to transmit one or more of possible communication methods corresponding to the selected user identifier, an address corresponding to the selected user identifier, and one or more of port numbers corresponding respectively to the possible communication methods corresponding to the selected user identifier to the terminal (102∼107) that transmits the request; and means for causing the terminal (102∼107) that transmits the request to establish, when a possible communication method to be started is included in the transmitted one or more of possible communication methods from the switchboard, a communication with a terminal (102∼107) corresponding to the selected user identifier using the transmitted address from the switchboard and a port number, the port number being included in the transmitted port numbers from the switchboard and corresponding to the possible communication method to be started, and to establish, when the possible communication method to be started is not included in the transmitted one or more of possible communication methods from the switchboard, a communication with the terminal (102∼107) corresponding to the selected user identifier using the transmitted address from the switchboard and a default port number corresponding to the possible communication method to be started. means for causing each of the terminals (102∼107) to register a user identifier, one or more of possible communication methods of the terminal (102∼107) itself, an address of the terminal (102∼107) itself corresponding to the possible communication methods, and one or more of port numbers corresponding respectively to the possible communication methods in the switchboard; means for, when one of the terminals (102∼107) transmits a request of transmitting a list of user identifiers connected to the switchboard, causing the switchboard to transmit the list to the terminal (102∼107) that transmits the request; means for, when the terminal (102∼107) that transmits the request selects one of the user identifiers from the list and transmits the selected user identifier to the switchboard, causing the switchboard to transmit one or more of possible communication methods corresponding to the selected user identifier, an address corresponding to the selected user identifier, and one or more of port numbers corresponding respectively to the possible communication methods corresponding to the selected user identifier to the terminal (102∼107) that transmits the request; and means for causing the terminal (102∼107) that transmits the request to establish, when a possible communication method to be started is included in the transmitted one or more of possible communication methods from the switchboard, a communication with a terminal (102∼107) corresponding to the selected user identifier using the transmitted address from the switchboard and a port number, the port number being included in the transmitted port numbers from the switchboard and corresponding to the possible communication method to be started, and to establish, when the possible communication method to be started is not included in the transmitted one or more of possible communication methods from the switchboard, a communication with the terminal (102∼107) corresponding to the selected user identifier using the transmitted address from the switchboard and a default port number corresponding to the possible communication method to be started.