Check multiple CIDR ranges for overlapping IP address spaces.
Last verified: April 2026
Output will appear here...The checker converts each CIDR notation into a numeric start-end range by parsing the IP address into a 32-bit integer and applying the prefix length as a bitmask. It then performs an O(n^2) pairwise comparison of all ranges, checking whether any range's start address falls within another range's span or vice versa. Overlaps are reported with the exact intersecting IP range and both source CIDR blocks for easy identification.
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.
Your company is connecting 12 AWS accounts via Transit Gateway and needs to add a Direct Connect link to the on-premises data center at 10.0.0.0/8. Before configuring the TGW routes, you paste all 12 VPC CIDRs plus the on-prem range into the checker. It flags that Account 7's VPC (10.42.0.0/16) overlaps with Account 3's secondary CIDR (10.42.0.0/20). Account 3 added that secondary CIDR months ago for an EKS cluster and nobody updated the IPAM spreadsheet. You catch it before the Transit Gateway route tables create a black hole.
Always check overlaps BEFORE creating VPC peering connections or Transit Gateway attachments. AWS will reject the peering request if CIDRs overlap, but by that point you may have already built infrastructure in the overlapping VPC. Changing a VPC's CIDR requires migrating everything — it's not an in-place operation.
Don't forget secondary CIDR blocks. A VPC might use 10.0.0.0/16 as its primary CIDR but have 10.1.0.0/16 as a secondary. If you only check primary CIDRs, you'll miss overlaps with the secondary ranges that will break routing just as badly.
For multi-cloud environments, check overlaps across ALL providers. Your AWS VPCs, Azure VNets, GCP VPCs, and on-premises networks all need non-overlapping address space if they'll ever be connected via VPN, Direct Connect, ExpressRoute, or Cloud Interconnect.
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.
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.
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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