'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].'

@classmethod
def _parse_hextet(cls, hextet_str):

    if (not cls._HEX_DIGITS.issuperset(hextet_str)):
        raise ValueError((u'Only    hex    digits    permitted    in    %r' % hextet_str))
    if (len(hextet_str) > 4):
        msg = u'At    most    4    characters    permitted    in    %r'
        raise ValueError((msg % hextet_str))
    return int(hextet_str, 16)
