Return prefix length from the bitwise netmask.

        Args:
            ip_int: An integer, the netmask in expanded bitwise format

        Returns:
            An integer, the prefix length.

        Raises:
            ValueError: If the input intermingles zeroes & ones

def _prefix_from_ip_int(self, ip_int):
        """Return prefix length from the bitwise netmask.

        Args:
            ip_int: An integer, the netmask in expanded bitwise format

        Returns:
            An integer, the prefix length.

        Raises:
            ValueError: If the input intermingles zeroes & ones
        """
        trailing_zeroes = _count_righthand_zero_bits(ip_int,
                                                     self._max_prefixlen)
        prefixlen = self._max_prefixlen - trailing_zeroes
        leading_ones = ip_int >> trailing_zeroes
        all_ones = (1 << prefixlen) - 1
        if leading_ones != all_ones:
            byteslen = self._max_prefixlen // 8
            details = _int_to_bytes(ip_int, byteslen, 'big')
            msg = 'Netmask pattern %r mixes zeroes & ones'
            raise ValueError(msg % details)
        return prefixlen