In addition to getting IP addresses through DHCP protocol, a client side software program and a serving side software program used by the method also keeps a communication session for used in accurately detecting the status information of client side such as: on line and off line information. The method features: the client side can initiates a DHCP request to alter IP address not depending the stipulation of address- reletting specified in the DHCP protocol; and meanwhile when the client side initiates the first and second request, it can identify the legal and illegal DHCP server according to the network attribute carried in DHCP message returned by the server, and only responds to legal server.