Method for sustaining NAT address in SIP network system and device thereof

The invention provides a NAT address binding and maintaining method and a corresponding device and system in a SIP network system. In the SIP network system, a SIP user agent is connected with a second network SIP server through a NAT/PAT device of a private network, the method comprises the following steps: a) the SIP server creates a NOTIFY information periodically to the SIP user agent; b) the SIP user agent responds to the NOTIFY information so as to maintain the binding of the NAT/PAT device to the NAT address. According to the method and the device provided by the invention, connectable performance of the SIP user agent in the private network can be effectively maintained, the reliability of the SIP network system is improved.