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If you’re working with cloud infrastructure and running into issues with subnet CIDR blocks, here’s a straightforward way to troubleshoot and fix the problem.
Suppose you’ve written a CloudFormation template to deploy your Virtual Private Cloud (VPC), subnets, security groups, and other resources. Your VPC is set with the IP range 172.16.0.0/12. This range covers addresses from 172.16.0.0 to 172.31.255.255, so you have a lot of room to create smaller subnets.
In your setup, you’ve divided this large block into smaller subnets like 172.28.0.0/15, aiming to use these segments for your resources. These smaller blocks fit within the larger /12 range, making them valid choices.
However, when you try to deploy the template, you receive an error saying the CIDR ‘172.28.0.0/15’ is invalid. This is surprising because 172.28.0.0/15 is within the 172.16.0.0/12 range, so what gives?
The root of this problem is that, even though the subnet CIDR appears correct, AWS has constraints on how small or specific these ranges can be. In AWS, certain CIDR blocks may be invalid if they don’t meet specific criteria or if overlaps with existing resources exist.
To fix this, check the following:
– Make sure the CIDR blocks you specify are within the overall VPC CIDR block.
– Confirm that your subnet CIDRs don’t overlap with other existing subnets.
– Use CIDR notation accurately, ensuring that the block size matches what you intend (for example, /15, /16, etc.).
– Double-check the AWS documentation to verify which CIDR blocks are allowed within your VPC’s range.
In this case, replacing /15 with a smaller or larger subnet, like /16, might solve the issue. For instance, try 172.28.0.0/16 instead of /15.
By carefully selecting your subnet ranges within the overall VPC CIDR block, and ensuring they don’t overlap, you’ll be able to deploy your infrastructure smoothly without encountering invalid CIDR errors.
If you follow these steps, you should be able to troubleshoot and resolve CIDR block issues in AWS CloudFormation.
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