a sip server having the function of accepting simultaneous ipv4 / ipv6 registrations from an ipv4 / ipv6 dual sip terminal , recognizing a protocol communicable with a communication partner terminal , and notifying the ipv4 / ipv6 dual sip terminals of the result of the recognition if necessary , thereby to allow voip communication between the ipv4 / ipv6 dual sip terminal and an ipv4 terminal or an ipv6 terminal to be the communication partner .