About this tool
Overlapping IP space is a common source of routing, VPN, NAT and provisioning problems. Two CIDR blocks overlap when at least one address belongs to both ranges. The prefixes do not have to be identical for that to happen.
This checker converts both networks to numeric start and end boundaries, then tests whether those ranges intersect. It is a quick way to catch conflicts before a change is built or a customer network is connected.
How to use it
- Enter Network A and Network B in CIDR notation.
- Calculate.
- If the result shows overlap, compare the start and end addresses and redesign or document the intended exception.
Example: 10.0.0.0/24 and 10.0.0.128/25
The /25 is entirely contained inside the /24, so the two networks overlap. A route or policy intended to treat them as unrelated address spaces would need additional design consideration.
Engineering notes
- Overlap may be intentional in route summarization, but it is usually a problem when two independently routed customer or infrastructure networks are expected to remain distinct.
- Check planned RFC1918 ranges during mergers, VPN integrations and cloud connectivity work; private-space collisions are especially common in those cases.
Frequently asked questions
Can one subnet be completely inside another?
Yes, and that is still an overlap. More-specific routes are commonly nested inside summaries.
Does overlap mean routing will always fail?
No. Routers use longest-prefix match, so nested routes can be deliberate. The problem is whether the overlap matches the intended design.
Should I check both public and private address space?
Yes. Any independently assigned networks can conflict if they are introduced into the same routing or translation domain.