def teredo(self):
        """Tuple of embedded teredo IPs.

        Returns:
            Tuple of the (server, client) IPs or None if the address
            doesn't appear to be a teredo address (doesn't start with
            2001::/32)

        """
        if (self._ip >> 96) != 0x20010000:
            return None
        return (IPv4Address((self._ip >> 64) & 0xFFFFFFFF),
                IPv4Address(~self._ip & 0xFFFFFFFF))