Implementing method of SIP application-level gateways based on NAT-PT

Disclosed is a method for applying a NAT-PT-based SIP onto the gateway to convert SIP message between an IPv4 network and an IPv6 network, comprising: obtaining the identification number and length of the SIP message; analyzing the first line of the message head to judge whether the type of the SIP message is a request or a response; if the answer is request, executing the request process; if theanswer is response, executing the response process. The invention can record the conversion of the IP address and the port via a conversion information table, manage the conversion state of the message by a state machine, and carry out the conversion from the IPv4 address to IPv6 address (or visa versa) based on an address prefixing and an address pool, accordingly, the conversion between the IPv4 network and IPv6 network is achieved.