This invention is to provide a method applied to a network system comprising Internet and at least two private networks each having at least one NAT router and at least one network terminal device. Each network terminal device can link to Internet through an ICE proxy and the NAT router in the corresponding private network. The method allows an ICE proxy in a private network to hijack connection signals sent from a network terminal device, to write a plurality of candidate access points provided by an ICE protocol standard into a SDP packet containing the connection signals, and to transmit the SDP packet to a remote ICE proxy in another private network via Internet. As a result, the ICE proxies of two private networks can selectively use the candidate access points provided by the ICE protocol standard in order to pass through the respective NAT routers and firewalls thereof.