 the communication node in the network protected by the firewall firstly sends a data packet to the mobile node outside the network protected by the firewall, the firewall will newly add an item in the entry item after the data packet traverses the firewall. The item includes such five elements as an address of the communication node, a new CoA of the mobile node, a port of the communication node, a new port of the mobile node, and a protocol number between the communication node and the mobile node. Therefore, when the 