Python script to represent a list of CIDR network ranges in the smallest number of CIDR subnets. Subnets can be added and substracted.

cidr_merge.py

#!/usr/bin/env python3

# Copyright 2018, 2026 SURF B.V.

# Requires python 3.10+

import argparse, sys
try:
    from netaddr import IPNetwork, IPSet
except ImportError:
    print("The netaddr module is required. Install it with e.g. 'pip install netaddr'.")
    sys.exit(1)


class IPNetworkArg:
    """Parse an IPv4 or IPv6 network argument from the command line
       The IP network can be any network that is supported by netaddr.IPNetwork.
       If no netmask is specified, /32 (for IPv4) or /128 (for IPv6) is assumed.
       E.g.:
         "192.168.0.0/24"
         "192.168.0.2" => "192.168.0.2/32"
         "2001:db8:abcd::/48"
         "2001:db8:abcd:1::10.0.0.1" => "2001:db8:abcd:1::a00:1/128"

       The network address can be prefixed with "+" (default) or "-" to specify whether the network
       must be added or removed from the range.

       Any leading/trailing whitespace and comments (# ...) are ignored.

       An empty string is assigned the None network.

    Supported """
    def __init__(self, network: str):
        # Remove comment (# ....) and leading/trailing whitespace
        network = network.split('#')[0].strip()

        if len(network)==0:
            self.__ip_network = None
            return

        self.__add = True
        if network.startswith('-'):
            network = network[1:]
            self.__add = False  # I.e remove
        elif network.startswith('+'):
            network = network[1:]

        try:
            self.__ip_network = IPNetwork(network)
        except Exception as e:
            raise argparse.ArgumentTypeError(f"Invalid IP network specification: \"{network}\". Error: {e}")

    @property
    def ip_network(self) -> IPNetwork | None:
        return self.__ip_network

    @property
    def add(self) -> bool:
        return self.__add

    def __str__(self):
        if self.__ip_network is None:
            return ""
        return ("+" if self.add else "-") + self.__ip_network.__str__()

    def __bool__(self):
        return self.__ip_network is not None


parser = argparse.ArgumentParser(
    prefix_chars='/',
    fromfile_prefix_chars='@',
    description='Represent a list of CIDR network ranges in the smallest number of CIDR subnets. '
                'The subnets are added to and subtracted from the final range in the order they are specified on the command line. '
                'Example: cidr_merge.py 192.168.0.0/28 -192.168.0.10'
)
parser.add_argument(
    dest='cidr_list',
    type=IPNetworkArg,
    nargs='+',
    help='A list of IP Addresses or IP networks in CIDR format (e.g. "192.168.0.0/24"). '
         'To subtract a range, prefix it with "-" (e.g. "-192.168.0.3"). '
         'To load a list from file use "@filename".'
         'Leading/trailing whitespace and comments (# ...) are ignored.',
)

# Show help and exit when no arguments are provided
if len(sys.argv)==1:
    parser.print_help()
    parser.exit(0)

# Parse arguments
args = parser.parse_args()

# Merge the CIDR ranges
merged_list=IPSet()
for cidr in args.cidr_list:
    if not cidr:
        continue

    if cidr.add:
        merged_list.add(cidr.ip_network)
    else:
        merged_list.remove(cidr.ip_network)

# Print merged list
for cidr in merged_list.iter_cidrs():
    print(cidr)