Never Make VPCs With Overlapping CIDRs

If you’re making new VPCs in your AWS account, don’t make VPCs with CIDR ranges that overlap with other VPCs.

If you don’t, you might run into the following issues:

• Routing - You can run into issues routing requests to the correct network.
• Security - It’s much more difficult to audit and review security groups based on CIDR blocks.
• VPN issues - Your VPN software can also run into issues routing when services are on the same CIDR block.
• Observability - When all your IPs are in the same CIDR block, observing and debugging your services is more difficult when all the logs are tagged with the same IP block.
• Compliance - Certain compliance frameworks require clear segmentation of network resources.

For these reasons, I recommend you never make VPCs with overlapping CIDRs.

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