To make it possible to transit a public telephone exchange network in which a communication technique of existing switching equipment is employed into an IP communication network using a connection-less type communication technique by providing a communication system that assures a communication speed, communication quality, and measures for communication failure in an integrated fashion to maintain security and reliability of the IP communication network while hardly changing a private address system and achieving communication between IP telephones using a telephone number based on an international telephone number system.

In a communication system, an IP communication network includes a domain name server. Terminals T1 and T2 are connected with the IP communication network. The domain name server receives a telephone number that is based on the international telephone number system of the terminal T2 from the terminal T1 and identifies the IP address of the terminal T2 corresponding to the telephone number to return the IP address to the Terminal T1. Using the IP frame containing the IP address, communication is performed between the terminal T1 and terminal T2 via the IP communication network.