Convert an IPv6 hextet string into an integer.

        Args:
            hextet_str: A string, the number to parse.

        Returns:
            The hextet as an integer.

        Raises:
            ValueError: if the input isn't strictly a hex number from
              [0..FFFF].

def _parse_hextet(cls, hextet_str):
        """Convert an IPv6 hextet string into an integer.

        Args:
            hextet_str: A string, the number to parse.

        Returns:
            The hextet as an integer.

        Raises:
            ValueError: if the input isn't strictly a hex number from
              [0..FFFF].

        """
        # Whitelist the characters, since int() allows a lot of bizarre stuff.
        if not cls._HEX_DIGITS.issuperset(hextet_str):
            raise ValueError("Only hex digits permitted in %r" % hextet_str)
        # We do the length check second, since the invalid character error
        # is likely to be more informative for the user
        if len(hextet_str) > 4:
            msg = "At most 4 characters permitted in %r"
            raise ValueError(msg % hextet_str)
        # Length check means we can skip checking the integer value
        return int(hextet_str, 16)