Abstract text

We propose a method for predicting a port number of a NAT equipment according to results of inquiring a STUN server twice in this invention. A network terminal device A in a private network sends an inquiry packet to the STUN server before and after sending a hole punching request, acquires two endpoint mappings of the NAT equipment used respectively in the two inquiries from the STUN server, and delivers the endpoint mappings to another network terminal device B in another private network through a SIP server. The network terminal device B predicts a range of port numbers of the NAT equipment probably used when the hole punching request was sent, according to the endpoint mappings and a port differential of the NAT equipment, and sends packets to the predicted port numbers sequentially until a response packet is received from the network terminal device A. Hence, a connection channel is established.