Check if an address is in a given subnet

subnetcheck.py

#! /usr/bin/env python3

split_fields = lambda addr: [int(f) for f in addr.split(".")]

def check_subnet(prefix_addr: str, prefix_length: int, to_check: str):
    """
    Determine if the IPv4 address to_check is within the given subnet

    :param prefix_addr: address prefix as shown in CIDR notation
    :param prefix_length: the number of bits in the network section 
    of the IP (subnet mask, as per CIDR notation)
    :param to_check: the IP address to check if within the given subnet

    :return: True if to_check is in the subnet specified by prefix_addr and prefix_length, else False
    """
    # split apart the bytes making up the IPv4 address
    pref_fields = split_fields(prefix_addr)
    check_addr_fields = split_fields(to_check)

    # assert valid IPs - each field limited to 8 bits
    for field in pref_fields + check_addr_fields:
        assert field < 256

    # identify the first varying field and the number of bits that are the same
    bits_to_check = prefix_length % 8
    field_to_check = int(prefix_length / 8)

    # confirm fully matching bytes do in fact match
    if pref_fields[:field_to_check] != check_addr_fields[:field_to_check]:
        return False

    # check that the network bits in the first varying field match
    shift = 8 - bits_to_check
    return (pref_fields[field_to_check] >> shift) == (check_addr_fields[field_to_check] >> shift)


if __name__ == '__main__':
    # is 10.0.65.13 in subnet 10.0.64.0/18?
    assert check_subnet("10.0.64.0", 18, "10.0.65.13")
    assert check_subnet("192.168.1.0", 24, "192.168.1.112")
    assert not check_subnet("192.168.1.0", 24, "192.168.2.2")