A network access device (10) for using in a mobile communication system includes a receiving module (100), a parsing module (110), a determining module (120), a storage module (130), and a sending module (140). The receiving module receives a packet, and the packet includes a source address and a destination address. The parsing module connected to the receiving module parses the packet to retrieve the source address and the destination address of the packet. The determining module connected to the parsing module determines whether a source of the packet has been authenticated. The storage module connected to the determining module saves notification messages indicating that the source of the packet has been authenticated. The sending module sends the packet to a destination corresponding to the destination address of the packet when the source of the packet has been authenticated.