impl/python/netmask_to_prefix.py

from .net_ipv4_parse_ipv4 import parse_ipv4


def netmask_to_prefix(netmask: str) -> int:
    """The prefix length of a dotted-quad mask; a mask with a gap in its one bits is refused."""
    value = parse_ipv4(netmask)
    host_part = 4294967295 - value + 1
    for prefix in range(33):
        if 2 ** (32 - prefix) == host_part:
            return prefix
    raise ValueError('"%s" is not a contiguous netmask' % (netmask,))