Patent ID: 7308495

Claim:
A network system comprising 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, wherein the router has: a first management unit configured to manage one or a plurality of first type prefixes and one or a plurality of first associated valid periods, and one or a plurality of second type prefixes that have shorter valid periods than the first type prefixes and one or a plurality of second associated valid periods, which are allocated from the provider; a request unit configured to request allocation of one or a plurality of new second type prefixes to the provider, before all of the second type prefixes managed by the management unit become invalid; and an advertisement unit configured to advertise periodically the first type prefixes that are still within said one or the plurality of first associated valid periods and information regarding the valid periods of the first type prefixes that is to be informed to the hosts, and the second type prefixes that are still within said one or the plurality of second associated valid periods and information regarding the valid periods of the second type prefixes that is to be informed to the hosts; and each host has: a generation unit configured to generate one or a plurality of first type IP addresses and one or a plurality of second type IP addresses, respectively according to the first type prefixes and the second type prefixes advertised from the router in an identical link; a second management unit configured to manage the first type IP addresses and the second type IP addresses generated by the generation unit, and valid periods of the first type IP addresses and the second type IP addresses that are determined according to latest information regarding the valid periods of the first type prefixes from which the first type IP addresses are generated and latest information regarding the valid periods of the second type prefixes from which the second type IP addresses are generated, which are advertised from the router; and a selection unit configured to select one IP address to be used as a source address of the each host from the first type IP addresses and the second type IP addresses that are still within said one or the plurality of second associated valid periods, at a start of communications by the each host.