Why do overlapping CIDRs cause problems?

When two networks share overlapping address space, routers cannot determine which network a packet should be forwarded to, resulting in dropped traffic or misrouted packets. AWS explicitly blocks VPC peering between VPCs with overlapping CIDRs.

Can I use RFC 1918 addresses in cloud VPCs?

Yes. The three RFC 1918 private ranges (10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16) are the most common choices for VPC address spaces. Most organizations standardize on 10.0.0.0/8 and carve it into smaller blocks per account or environment.

How does this tool determine overlap?

The tool converts each CIDR block to its start and end IP addresses as integers, then checks whether any range's start falls within another range or vice versa. Two ranges overlap if and only if one range's start address is less than or equal to the other range's end address and greater than or equal to its start address.

CIDR Overlap Checker

NetworkingAWS

Check multiple CIDR ranges for overlapping IP address spaces.

CIDR Ranges

Raw Output

Output will appear here...

About This Tool

The CIDR Overlap Checker takes multiple CIDR blocks and determines whether any of them share overlapping IP address space. Overlapping ranges are a common source of routing failures, VPC peering rejections, and VPN tunnel misconfigurations. This tool performs pairwise comparison of all entered ranges and highlights every conflict, saving engineers from manually converting blocks to binary for comparison.

When to Use This Tool

•Validating that a new VPC CIDR block does not overlap with existing VPCs before establishing peering connections.
•Checking on-premises network ranges against cloud VPC ranges before configuring a site-to-site VPN or Direct Connect.
•Auditing multi-account AWS Organizations to ensure no two accounts use conflicting address spaces.
•Verifying Transit Gateway route table entries do not contain overlapping destination CIDRs.

Frequently Asked Questions

Why do overlapping CIDRs cause problems?: When two networks share overlapping address space, routers cannot determine which network a packet should be forwarded to, resulting in dropped traffic or misrouted packets. AWS explicitly blocks VPC peering between VPCs with overlapping CIDRs.
Can I use RFC 1918 addresses in cloud VPCs?: Yes. The three RFC 1918 private ranges (10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16) are the most common choices for VPC address spaces. Most organizations standardize on 10.0.0.0/8 and carve it into smaller blocks per account or environment.
How does this tool determine overlap?: The tool converts each CIDR block to its start and end IP addresses as integers, then checks whether any range's start falls within another range or vice versa. Two ranges overlap if and only if one range's start address is less than or equal to the other range's end address and greater than or equal to its start address.

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Disclaimer: This tool runs entirely in your browser. No data is sent to our servers. Always verify outputs before using them in production. AWS, Azure, and GCP are trademarks of their respective owners.