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Asked: 2024-09-27T12:32:32+05:30 In:

How can I efficiently simplify overlapping CIDR blocks into the fewest possible subnets in Python?

I’ve been trying to wrap my head around creating a subnet cleaner for IPv4 addresses, especially in CIDR notation, and it’s driving me a bit crazy! So, here’s the deal: I have this list of subnet masks in CIDR format, and I want to simplify it into the fewest possible CIDR blocks. The challenge is that sometimes the subnets overlap, or some might be completely contained within others.

Let’s say I have the following list to start with:

– 192.168.1.0/24
– 192.168.1.128/25
– 192.168.2.0/24
– 192.168.1.0/25
– 192.168.1.64/26

When I look at these, I can see that there’s a lot of overlap, and it feels like I’m missing some key insight to condense these down. For instance, the first two subnets seem to overlap quite a bit, as 192.168.1.0/24 covers everything from 192.168.1.0 to 192.168.1.255, right? And then 192.168.1.128/25 only covers from 192.168.1.128 to 192.168.1.255. Assuming I can clean that up, would I end up with just 192.168.1.0/24 and 192.168.2.0/24, or is there a more efficient way to group these?

What about 192.168.1.0/25 and the /26? Shouldn’t they also be folded in to the /24? And then how would I go about implementing this in a programming language, say Python or even something lower level like C?

I guess what I’m really asking is: what’s a good algorithmic approach to take for simplifying these CIDR ranges without losing any coverage? And could anyone share some code snippets or logic that could help me get started? Any help would be super appreciated, or if you just wanna talk subnet logic, I’m all ears!

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    2 Answers

    1. CIDR Block Simplifier

      Here’s a simple approach to condense overlapping CIDR blocks into the fewest possible CIDR blocks:

      
import ipaddress

def simplify_cidrs(cidr_list):
    # Create a set to hold the unique networks
    all_networks = set()

    # Convert CIDR blocks to network objects
    for cidr in cidr_list:
        network = ipaddress.ip_network(cidr, strict=False)
        all_networks.add(network)

    # Combine overlapping networks
    combined_networks = []
    while all_networks:
        # Pop a network from the set to start combining
        current_network = all_networks.pop()
        combined = [current_network]

        # Check for overlaps with other networks
        for other_network in list(all_networks):
            if current_network.overlaps(other_network):
                combined.append(other_network)
                all_networks.remove(other_network)

        # Create a union of found networks
        combined_network = ipaddress.collapse_addresses(combined)
        combined_networks.extend(combined_network)

    return combined_networks

# Sample CIDR blocks
cidrs = [
    '192.168.1.0/24',
    '192.168.1.128/25',
    '192.168.2.0/24',
    '192.168.1.0/25',
    '192.168.1.64/26'
]

# Simplify the CIDR blocks
simplified = simplify_cidrs(cidrs)
print("Simplified CIDR Blocks:")
for cidr in simplified:
    print(cidr)

      This code uses Python’s ipaddress module to handle the CIDR blocks and determine overlaps. The algorithm starts by creating a set of unique networks and then iteratively combines overlapping networks into the simplest possible form. It’s a straightforward approach and should give you the results you’re looking for!

      Feel free to tweak the code to fit more specific needs or to add additional features like input validation!

    2. To simplify your list of CIDR blocks, you can take the approach of merging overlapping and contained subnets iteratively. The algorithm involves sorting the CIDR blocks based on their starting address and then checking for overlaps. If a block completely overlaps another, you can remove the smaller one. For your given example, after processing the list, you would indeed end up with two CIDR blocks: 192.168.1.0/24 and 192.168.2.0/24. The 192.168.1.128/25 and 192.168.1.0/25 are covered by the 192.168.1.0/24, and the 192.168.1.64/26 is contained within it as well.

      Here’s a Python snippet to help you get started with the simplification process:

      
import ipaddress

def simplify_cidrs(cidrs):
    # Convert strings to ip_network objects
    networks = [ipaddress.ip_network(cidr) for cidr in cidrs]
    # Sort by network address
    networks.sort()
    
    simplified = []
    current = networks[0]
    
    for net in networks[1:]:
        if current.overlaps(net):
            current = current.supernet(new_prefix=current.prefixlen)  # Merge into current
        else:
            simplified.append(current)
            current = net
    simplified.append(current)  # Append last network
    
    return [str(net) for net in simplified]

# Example CIDRs
cidr_list = [
    "192.168.1.0/24",
    "192.168.1.128/25",
    "192.168.2.0/24",
    "192.168.1.0/25",
    "192.168.1.64/26"
]

# Simplifying CIDRs
result = simplify_cidrs(cidr_list)
print(result)  # Output: ['192.168.1.0/24', '192.168.2.0/24']

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