In a network system formed by one or a plurality of hosts having a function for accessing Internet, and a router for connecting each host and the Internet through an accessing communication path of a provider, two types of IP address including first type IP addresses and second type IP addresses are generated and selectively used by each host, according to two types of prefix including first type prefixes and second type prefixes that have shorter valid periods than the first type prefixes, which are advertised from the router periodically.