Wherein, the access device adds its own relay agent ID and the port identifier corresponding to user's device into the router request message sent by user's device by an approach of newly defining option, and sends it to a corresponding edge router; the edge router configures the user's device with a dedicated address prefix according to the relay ID and the port identifier, and combines the relay ID and the port identifier into a router response message that will be sent to the access device represented by the relay ID; the access device sends the router notice message only including the address prefix to the user's device through the line port represented by the port identifier.