In order to constitute a domain name system obtained by providing and utilizing a DNS service by using a self-supporting (not rented) DDNS server in a dynamic IP address environment, a DDNS server inside a network of a central site and routers which are DDNS clients inside networks of other sites are provided. The DDNS server includes: a means for registering a relationship between domain names of the DDNS clients and global addresses of the DDNS client; a means for responding to a request for resolving a name from the DDNS client; and a means which, at a time when a global address of the DDNS server is updated, notifies DDNS server information including the global address of the DDNS server to the DDNS client. Moreover, the DDNS client includes: a means for registering (updating) the global address of the DDNS server when the DDNS server information is notified; and a means for issuing a request for resolving a name to the DDNS server.