About CIDR Overlap Checker

Checks a list of IPv4 CIDR blocks and addresses for overlaps — the conflicts that cause shadowed firewall rules, ambiguous routes, and duplicate security-group entries. Every pair of blocks that shares any address is reported, with the exact relationship (identical/duplicate, one fully contains the other, or partial overlap) and your own labels carried through so you can see which rules collide. Blocks that do not share any address are correctly reported as disjoint.

• Compares every pair of entries by address range, not just by text
• Classifies each overlap: identical/duplicate, A-contains-B, B-contains-A, or partial
• Accepts bare IPs (treated as /32) and CIDR blocks /0–/32, with optional per-line labels
• Labels are preserved in the report so colliding rules are easy to identify
• Invalid lines are skipped and listed rather than failing the whole batch
• Runs entirely in the browser — no addresses leave your machine

Frequently Asked Questions

What counts as an overlap?

Any two entries that share at least one address. For aligned CIDR blocks that means one contains the other or they are identical; bare IPs overlap a block when they fall inside it. The report names the exact relationship for each conflicting pair.

Why does this matter for firewall and NSG rules?

Overlapping CIDRs in an allow/deny list create shadowed or ambiguous rules — a broader rule can mask a narrower one, or two rules can contradict. Surfacing the overlaps lets you spot duplicates and ordering problems before they cause an outage or a security gap.

Can I label my entries?

Yes. Put a label after the CIDR on the same line (e.g. 10.0.0.0/24 prod-web) and it is carried into the overlap report so you can tell which named rules collide.