CIDR Overlap Checker
Compare two IPv4 CIDR blocks and calculate their shared interval.
Detect IPv4 subnet intersections
Two CIDR blocks overlap when the greater starting address is no higher than the smaller ending address. This checker parses and normalizes both IPv4 networks, computes that interval comparison, and reports either no intersection or the exact shared first address, last address, and address count. Host bits in a typed network address are masked before the ranges are compared.
The default /24 and its upper /25 overlap from 192.168.1.128 through 192.168.1.255, covering 128 addresses. Two adjacent blocks whose end and start differ by one return no because adjacency is not overlap. A malformed prefix or octet produces a direct validation error rather than a guessed correction.
Intersections, adjacency, and route policy
Range overlap can reveal conflicting address plans, firewall objects, VPN routes, or cloud networks, but it does not determine which route wins. Longest-prefix matching and administrative policy still control forwarding. The calculation is private in your browser and performs no network discovery. Compare every relevant allocation, including translated and tunneled ranges, before changing a production topology.
Frequently Asked Questions
Are adjacent CIDR blocks considered overlapping?
No. They can touch at a boundary without sharing any address.
Does the output show the shared address count?
Yes. An overlapping result includes the inclusive shared range and its exact number of addresses.
Can overlap determine route priority?
No. Route selection depends on prefix specificity and policy beyond the mathematical intersection.
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