Containment · duplicates · IPv4 · IPv6
CIDR Overlap Checker
Paste a list of networks. Every pair where one block contains another, or two are identical, is listed with the line numbers. Invalid lines are reported, and the result says "incomplete" instead of claiming a clean list.
Networks to check
One per line, or separated by commas. Masks and wildcards work too.
Result
Pairs are reported as line A relation line B, using the line numbers on the left.
- line 1 10.0.0.0/24 contains line 2 10.0.0.128/25
Found a pair? Check the parent block in the subnet calculator, or split it with the VLSM planner.
What the check reports
CIDR blocks are aligned ranges, so two of them can only relate in three ways: they are identical, one contains the other, or they are disjoint. This checker lists every pair in the first two groups and counts the networks it examined, so "no overlaps" is a statement about a complete list rather than a guess.
Common causes of overlaps
- A summary route next to its components. Advertising
10.0.0.0/16and10.0.5.0/24together is an overlap, because the /24 is inside the /16. - A subnet carved twice. The same
10.0.1.0/24on two lines usually means two teams were handed the same block. - Cloud ranges that were sized independently. VPC and VNet peering fails or behaves unpredictably when the two address spaces overlap.
- A broad prefix that swallowed a later allocation. A
/22planned first can contain several later/24s if the plan was not checked.
What to do about an overlap
If one block contains another by design, the pair is usually a summary route and its more specific route; keep both only when the routing plan calls for it. Otherwise, renumber one side. The VLSM planner can repack a parent block so the subnets no longer collide, and the CIDR tools page can aggregate a cleaned-up list. For turning a start and end address into blocks, use the range to CIDR converter.
Questions
What counts as an overlap between two CIDR blocks?
Two CIDR blocks either do not overlap at all or one fully contains the other. There is no partial overlap: if the blocks share even one address, the smaller block lies entirely inside the larger one. Identical blocks are also reported.
Why do overlapping subnets matter?
An overlap usually means a duplicate route, a VPC or VNet peering conflict, or a subnet that was carved out twice. Routers pick one of the two paths, so traffic can go somewhere you did not plan.
Does the checker find duplicate networks?
Yes. Two identical blocks are reported as equal. A network that appears twice with the same prefix, such as 10.0.0.0/24 listed on two lines, is flagged as an overlapping pair.
Can I check IPv4 and IPv6 together?
Yes, you can paste both in one list. IPv4 and IPv6 blocks can never overlap each other, so cross-family pairs are not reported; overlaps are only checked within the same IP version.