According to one embodiment, there is provided a communication system including at least a router and a server. The router interconnects a private network using private address and a global network using global address. The router has a network address translation (NAT) function for converting the private address into the global address and for converting the global address into the private address. The server unit is coupled to the global network and executes processing based on Session Initiation Protocol (SIP) messages output from a terminal via the router. One type of SIP message is a REGISTER message, where the router adds a Via header containing an Internet Protocol (IP) address of the router to the REGISTER message for designating a transmission route for use in a registration process.