A network system and a communication method are provided to solve an NAT problem to functionalize a P2P application in case that new port information (number is assigned to an NAT device. One communication device sends a pseudo-packet containing an address of another communication device, to a port information transmitting device via an address translation device; the port information transmitting device sends a response to the received pseudo-packet to the communication device; and the address translation device assigns new port information corresponding to the communication device address using the response and sends the address of the communication device and the assigned port information to the communication device. The communication device sends the received address of the communication device and the assigned port information to another communication device; and another communication device transmits information with the communication device using the received address of the communication device and the assigned port information.