Patent ID: 8295281

Claim:
A communication system comprising: a first communication terminal connected to an internal communication network including a communication channel used for transmission of communication packets that comprise a data area and a header area, the data area comprising an Internet Protocol (IP) address and a port number of a transmission source, the header area comprising an IP address and a port number of a transmission source and a transmission destination; a second communication terminal connected to an external communication network including the communication channel and arranged in an area which is larger than the internal communication network; and a connection device configured to connect the internal communication network and external communication network to each other, and to convert the IP address of the transmission source included in the header area into an IP address used on the external communication network when the communication packets are received from the internal communication network, and to convert the IP address of the transmission source included in the header area into an IP address used on the internal communication network when the communication packets are received from the external communication network; wherein the second communication terminal includes: a memory configured to store an IP address and a port number included in a data area of a control signal transmitted from the first communication terminal, when the communication channel to connect the first communication terminal and the second communication terminal is determined; a comparing module configured to compare the IP address stored in the memory with the IP address included in the header area when a first communication packet is input via the external communication network, and to further compare the port number stored in the memory with the port number included in the header area of the first communication packet if the IP address stored in the memory does not coincide with the IP address included in the header area of the first communication packet; and a processor configured to add the IP address and port number stored in the memory to a second communication packet to be transmitted to the first communication terminal if the IP address stored in the memory and the IP address included in the header area of the first communication packet coincide with each other, and to add the IP address included in the header area of the first communication packet to the second communication packet if the IP address stored in the memory and the IP address included in the header area of the first communication packet do not coincide with each other but the port number included in the header area of the first communication packet and the port number stored in the memory do coincide with each other.