이거 약간 수정해서 알수 있네요. ppp사용자 일 경우 유용할듯.

/* call WSAStartup first */
        WSAData wsaData;
         char   *Address;
      if (WSAStartup(MAKEWORD(1, 1), &wsaData) != 0)
               return "";



         SOCKET sd = WSASocket(AF_INET, SOCK_DGRAM, 0, 0, 0, 0);
     INTERFACE_INFO InterfaceList[20];
     unsigned long nBytesReturned;
     if ( WSAIoctl(sd, SIO_GET_INTERFACE_LIST, 0, 0,&InterfaceList,sizeof InterfaceList, &nBytesReturned, 0, 0) != 0)
     {
         return "";
     }
     int nNumInterfaces = nBytesReturned / sizeof(INTERFACE_INFO);
     for (int i = 0; i < nNumInterfaces; i++) {
           INTERFACE_INFO* ii = &InterfaceList[i];

      Address=inet_ntoa(ii->iiAddress.AddressIn.sin_addr);
      }
      return Address;
}