Patent ID: 7797568

Claim:
A relay agent device disposed in one of a plurality of subnets, each subnet containing terminal devices and an address leasing device, the address leasing device performing an address lease service for the terminal devices, comprising: a notifying unit notifying a proxy address leasing device, disposed in a network employed in common between the plurality of subnets, of address lease state information about the terminal devices, each address lease state information containing an IP (Internet Protocol) address of a terminal, an IP address of the relay agent device, and a MAC address of the terminal and lease time; a detecting unit detecting a fault in the address leasing device; and a transfer unit transferring a message related to the address lease, received from the terminal device in the subnet, to the proxy address leasing device in the network, and transferring a message related to the address lease, which is addressed to the terminal in the subnet from the proxy address leasing device in the network, to the terminal in the subnet, in order for the proxy address leasing device in the network to take over an IP address lease service provided by the address leasing device in the subnet when the fault is detected.