Patent ID: 7801059

Claim:
A NAT type determination method for determining a NAT type, comprising: a first transmission test, in which an IP communication apparatus that performs communication via a relay apparatus having a NAT function, transmits a first request to a first destination of a STUN server, the first destination including a first IP address and a first port number; a second transmission test, in which the IP communication apparatus transmits to the first destination, a second request that requests a change of a source IP address and a port number in a response; a third transmission test, in which the IP communication apparatus transmits a third request to a second destination of the STUN server, the second destination including a second IP address and a second port number; a fourth transmission test, in which the IP communication apparatus transmits to the first destination, a fourth request that requests a change of only a source port number in a response; and a fifth transmission test, in which the IP communication apparatus transmits a fifth request to a third destination, which includes at least one of a third IP address and a third port number different from the first and second destinations, wherein when finishing the first, second, third, fourth, and fifth transmission tests to the first, second, and third destinations at least one time each, the IP communication apparatus compares first source address information of one of the first, second, third, fourth, and fifth requests added in a first response from the STUN server in response to the one of the first, second, third, fourth, and fifth requests issued in a latest performed transmission test of the first, second, third, fourth, and fifth transmission tests, and second source address information of another of the first, second, third, fourth, and fifth requests added in a second response from the STUN server in response to the another of the first, second, third, fourth, and fifth requests issued in a preceding transmission test of the first, second, third, fourth, and fifth transmission tests, and when the first and second source address information are not identical, the IP communication apparatus determines that the NAT type is symmetric.