IP communication apparatus and IP communication method of such apparatus

An IP terminal apparatus has a SIP controller, a UDP packet controller, and an RTP controller. The SIP controller transmits and receives a call control message in order to establish a connection to and from a destination apparatus. The UDP packet controller transmits and receives media data with the destination apparatus in a media session after the connection was established. The RTP controller obtains source port number information of a media data packet that the UDP packet controller has received from the destination apparatus, and sets the obtained source port number as a destination port number for a packet to be transmitted to the destination apparatus. Thereby, a feasible range of NAT traversal can be improved in a simple and secure manner, even when at least one relay apparatuses employs symmetric NAT.