Exact duplicate
Two entries resolve to the same first and last address. Consolidate their ownership and purpose before treating them as separate objects.
CIDR Overlap Checker
Paste planned IPv4 CIDRs or explicit address ranges. The checker normalizes their boundaries and reports every pair that shares address space, including exact duplicates and one range contained inside another.
The list is processed locally. Comments beginning with # are ignored.
Compare planned address space
Enter one IPv4 CIDR or increasing start-to-end range per line. Labels can follow a space or vertical bar so findings remain connected to the design.
Examples: 10.20.0.0/16 office or 10.20.8.1 - 10.20.8.30 | temporary pool
Overlap is a design fact, not automatically an error
A smaller subnet normally sits inside a larger aggregate. That may be intentional in routing, but the same relationship can break VPN connectivity, cloud peering, address translation, or firewall object assumptions when separate zones are expected to be unique.
Contained range
Every address in one entry already belongs to another. Decide whether the narrower object exists for policy precision or is redundant.
Partial overlap
Explicit address ranges can intersect without full containment. That often signals an allocation error because ownership changes inside both ranges.
Resolve conflicts at the authoritative source
The tool compares mathematics only. It does not know which DHCP scope, VLAN, route, VPN, cloud network, or firewall object is authoritative. Record the owner and intended boundary for every flagged pair, then correct the address plan before copying the change into several systems.
Decide which record controls
For an allocation conflict, compare the IP address management record, approved design, active route, and lease configuration. Name one source of truth and one owner for the correction. A mathematically clean spreadsheet can still disagree with production, so preserve the before state and verify live paths after the change.
Classify the overlap before changing addresses
| Finding | Likely design question | Evidence to collect |
|---|---|---|
| Aggregate contains a subnet | Is the smaller block an intentional routing or policy object? | Route purpose, owner, interface, and policy references |
| Two independent sites overlap | Will a VPN or merger need both ranges at the same time? | Current leases, NAT plan, migration window, and rollback |
| Explicit pools partially overlap | Did two teams allocate the same addresses? | IPAM history, DHCP scopes, reservations, and approvals |
- Freeze new assignments in the disputed space.
- Confirm active use from authoritative leases, routes, and device records.
- Choose the range that will move and document every dependent object.
- Test routing and blocked paths after the change, then update the source of truth.
Continue with exact boundaries
Use the subnet calculator to inspect either range in detail. If the issue is an oversized policy list rather than a design collision, use the IP allowlist cleaner to remove duplicate and covered entries without granting new space. Record affected devices in the network inventory builder before a renumbering project.
CIDR aggregation and prefix behavior are described in RFC 4632. The displayed overlap does not prove live routing or reachability.