Communication apparatus, communication system, control method of communication apparatus and program

The present invention discloses a communication apparatus, a communication system and a control method of the communication apparatus. The communication apparatus includes a transmission source mapping algorithm acquisition unit configured to acquire an algorithm that assigns an external address and an external port number to an internal address and an internal port number of a transmission source, as a transmission source mapping algorithm, a destination mapping algorithm acquisition unit configured to acquire an algorithm that assigns an external address and an external port number to an internal address and an internal port number of a destination, as a destination mapping algorithm, and a generation unit configured to generate an external port number newly assigned to the internal address and the internal port number of the destination in a case where the transmission source mapping algorithm is more complicated than the destination mapping algorithm. The control method comprises the steps of acquiring a