A communication system, for functioning without the use of dedicated lines or the Internet so as to ensure communication speed, communication quality, and communication trouble countermeasures, including a communication network and domain name server. The domain name server includes a domain name tree with a country number of a telephone number as a level 2 domain name of the domain name tree, and the domain name server receives, from a terminal, a telephone number of a destination terminal. Furthermore, based on the telephone number of the destination terminal, the domain name server (i) seeks out, in the domain name tree, an Integrated Information Communication System (ICS) user address of the destination terminal, and (ii) sends the ICS user address to the terminal, such that the communication system receives, from the terminal, the ICS user address as a destination address, and sends the ICS user frame to the destination terminal.