'Return prefix length from a numeric string
Args:
prefixlen_str: The string to be converted
Returns:
An integer, the prefix length.
Raises:
NetmaskValueError: If the input is not a valid netmask'

@classmethod
def _prefix_from_prefix_string(cls, prefixlen_str):

    if (not _BaseV4._DECIMAL_DIGITS.issuperset(prefixlen_str)):
        cls._report_invalid_netmask(prefixlen_str)
    try:
        prefixlen = int(prefixlen_str)
    except ValueError:
        cls._report_invalid_netmask(prefixlen_str)
    if (not (0 <= prefixlen <= cls._max_prefixlen)):
        cls._report_invalid_netmask(prefixlen_str)
    return prefixlen
