A router device for redistributing, to a node, a prefix distributed from a prefix distributing device to route a packet based on an IPv6 address, the router device comprising: a prefix management unit which is configured to create a filtering rule including an indication of passage or block of a packet from the node based on the prefix distributed from the prefix distributing device and a prefix in a source IP address received from the node; a filter unit which is configured to pass or block the packet from the node according to the filtering rule created by the prefix management unit; a prefix storage unit which is configured to store the prefix, from the prefix distributing device, together with discriminative data used to determine whether the prefix, in the source IP address, is usable; and a Router Advertisement, RA, sending unit which is configured to periodically check on contents stored in the prefix storage unit, create option information on an RA packet capable of identifying whether a prefix, in the source IP address, is usable or unusable based on the prefix, from the prefix distributing device, and the discriminative data stored in the prefix storage unit, and send another node an RA packet including the created option information; wherein when a received packet meets the indication of blocking the packet in the filtering rule, the filter unit is configured to discard the packet and notify the prefix management unit of the source IP address of the discarded packet, the prefix management unit is configured to determine that the prefix included in the notified source IP address is a prefix unusable and to be deleted, the indication of blocking the packet in the filtering rule is that the received packet includes, in the source IP address, the prefix which is not distributed to the node by the router device, and when the packet including the prefix in the source IP address is received from the node, the filter unit is configured to discard the packet. a prefix management unit which is configured to create a filtering rule including an indication of passage or block of a packet from the node based on the prefix distributed from the prefix distributing device and a prefix in a source IP address received from the node; a filter unit which is configured to pass or block the packet from the node according to the filtering rule created by the prefix management unit; a prefix storage unit which is configured to store the prefix, from the prefix distributing device, together with discriminative data used to determine whether the prefix, in the source IP address, is usable; and a Router Advertisement, RA, sending unit which is configured to periodically check on contents stored in the prefix storage unit, create option information on an RA packet capable of identifying whether a prefix, in the source IP address, is usable or unusable based on the prefix, from the prefix distributing device, and the discriminative data stored in the prefix storage unit, and send another node an RA packet including the created option information; wherein when a received packet meets the indication of blocking the packet in the filtering rule, the filter unit is configured to discard the packet and notify the prefix management unit of the source IP address of the discarded packet, the prefix management unit is configured to determine that the prefix included in the notified source IP address is a prefix unusable and to be deleted, the indication of blocking the packet in the filtering rule is that the received packet includes, in the source IP address, the prefix which is not distributed to the node by the router device, and when the packet including the prefix in the source IP address is received from the node, the filter unit is configured to discard the packet.