Patent ID: 7796615

Claim:
A communication system for communication between a plurality of networks, comprising: a first local network having at least two, first and second communication terminals and connected to a first Network Address Port Translation (NAPT) device; a second local network having at least one, third communication terminal and connected to a second NAPT device; and a global network for connecting the first and second NAPT devices, the first and second communication terminals being respectively assigned with first and second local addresses which are effective only in the first local network, the third communication terminal being assigned with a third local address which is effective only in the second local network, the first and second NAPT devices being respectively assigned with first and second global addresses effective in the global network, the communication system further comprising: a main device having a table for storing identification information, a local address, and a global address of each of the communication terminals in association with each other, wherein: upon receiving a make-call request from the first communication terminal to make a call to the third communication terminal, the main device determines by referring to the table that the first communication terminal and the third communication terminal belong to different networks and uses the global addresses as mutual destination addresses; and upon receiving a make-call request from the first communication terminal to make a call to the second communication terminal, the main device determines by referring to the table that the first and second communication terminals belong to the same network and uses the local addresses as mutual destination addresses, wherein: when the main device determines that the first and third communication terminals belong to different networks, the main device creates data to be transmitted to the second NAPT device while using the first global address of the first NAPT device as a source address of the make-call request in the data; and when the third communication terminal responds to the make-call request, the main device creates data to notify the first communication terminal of that response while using the second global address of the second NAPT device as a address of the third communication terminal responding to the communication.