Patent ID: 7522618

Claim:
A communication apparatus connected to a server via a first relay apparatus having a NAT (network address translation) function, the communication apparatus comprising: an address information examination request transmission section for executing, in an arbitrary order, first transmission processing of transmitting at least two address information examination request packets having an identical source port number and different destination port numbers to the server and second transmission processing of transmitting at least one address information examination request packet having a source port number which is different from the source port number used in the first transmission processing to the server; a communication preparation request transmission section for transmitting a communication preparation packet having a source port number which is identical to the source port number used in either the first transmission processing or the second transmission processing to a destination which is different from the server, between the first transmission processing and the second transmission processing; an address information receiving section for receiving, from the server, at least three address information packets each including, in a data section thereof, a relay port number translated by the NAT function of the first relay apparatus from a source port address of the respective address information examination request packet; and a relay characteristic determination section for determining a NAT type of the first relay apparatus based on the relay port numbers included in the received address information packets.