Test if address is in network (in CIDR notation); IPv4 only, but IPv6 support should be easy

smack_my_IP_stack.py

import socket, struct

def ip_addr_to_bin_str(ip):
    # To support IPv6, play with socket.inet_pton.
    return format(struct.unpack('!I', socket.inet_aton(ip))[0], 'b')

def ip_in_cidr_net(ip_address, cidr_network):
    ip_bin = ip_addr_to_bin_str(ip_address)

    # Extract the "base" IP from the network address
    cidr_addr, cidr_size = cidr_network.split("/")
    netmask_bin = ip_addr_to_bin_str(cidr_addr)[:int(cidr_size)]

    return ip_bin.startswith(netmask_bin)


# ip_in_cidr_net('10.1.2.3', '10.0.0.0/8')    -> True
# ip_in_cidr_net('10.0.0.8', '10.0.0.0/24')   -> True
# ip_in_cidr_net('10.0.0.8', '10.0.0.78/24')  -> True
# ip_in_cidr_net('10.0.0.8', '10.8.8.345/24') -> raises socket.error, useful if you want to test IPs found in movies

# ip_in_cidr_net('192.168.100.1',   '192.168.100.0/22') -> True
# ip_in_cidr_net('192.168.103.255', '192.168.100.0/22') -> True
# ip_in_cidr_net('192.168.104.0',   '192.168.100.0/22') -> False