CIDR Overlap Checker
Detect overlapping network prefixes.
CIDR Overlap Checker: inputs and results
Enter an IP address, CIDR prefix, or range. Then check its first and last address below.
Calculated Boundaries & ParametersReady
Key Capabilities
- •100% Client-Side: Zero telemetry or server latency.
- •RFC Compliant: Exact 32-bit and 128-bit integer precision.
- •Quick Copy: Instant access to CIDR, mask, and range bounds.
- •Cross-Platform: Works on desktop, tablet, and mobile browsers.
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Subnet Boundary Checker
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IP-in-Range Checker
Test whether an address belongs.
IP-in-Subnet Checker
Test membership within a CIDR block.
How CIDR Overlap Checker Works?
This tool works by comparing 2 or more CIDR prefixes pair by pair and classifying each pair as identical, nested, or disjoint. CIDR blocks never partly overlap.
Paste the prefixes
Enter a list such as
10.0.0.0/16,10.0.128.0/17, and10.1.0.0/16.Take the shorter prefix
For each pair, the checker masks both networks with the shorter prefix length.
Compare the masked values
Equal values mean one block contains the other. Different values mean the blocks are disjoint.
List the conflicts
The checker lists every overlapping pair with its relationship.
CIDR Overlap Checker Formula
The formula is Overlap = (N_A AND Mask(min(p_A, p_B))) = (N_B AND Mask(min(p_A, p_B))).
Overlap test
mask both with min(p_A, p_B) and compareEqual → overlap.
Nesting rule
overlap ⇒ the shorter prefix contains the longer oneCIDR blocks are either nested or disjoint.
Pairs checked
n × (n − 1) ÷ 210 prefixes → 45 pairs.
Worked example: 10.0.0.0/16 vs 10.0.128.0/17
min prefix = 16 → mask 255.255.0.0 10.0.0.0 AND mask = 10.0.0.0 10.0.128.0 AND mask = 10.0.0.0 Equal → overlap, /16 contains /17
CIDR Overlap Checker Conversion Table
The table classifies 9 pairs of CIDR prefixes.
| Prefix A | Prefix B | Relationship | Overlap |
|---|---|---|---|
| 10.0.0.0/16 | 10.0.128.0/17 | A contains B | Yes |
| 10.0.0.0/16 | 10.1.0.0/16 | No overlap | No |
| 172.16.0.0/12 | 172.31.0.0/16 | A contains B | Yes |
| 192.168.0.0/24 | 192.168.0.0/24 | Identical | Yes |
| 192.168.1.0/25 | 192.168.1.0/24 | B contains A | Yes |
| 10.0.0.0/8 | 100.64.0.0/10 | No overlap | No |
| 192.168.0.0/22 | 192.168.4.0/22 | No overlap | No |
| 172.20.0.0/14 | 172.22.10.0/24 | A contains B | Yes |
| 0.0.0.0/0 | 10.0.0.0/8 | A contains B | Yes |
What is CIDR Overlap Checker?
The CIDR Overlap Checker is a tool that finds overlapping prefixes in a list of CIDR blocks and shows which block contains which.
CIDR blocks follow a strict property: 2 prefixes either do not overlap at all, or 1 contains the other completely. Partial overlap between 2 prefixes is impossible. The checker uses that property to give a precise answer: identical, A contains B, B contains A, or no overlap.
Overlaps among routed prefixes, VPC CIDRs, or firewall objects create ambiguous paths and duplicate rules. Spotting them early prevents routing conflicts.
What are the uses of CIDR Overlap Checker?
There are 4 main uses of the CIDR Overlap Checker, including VPC peering, route table hygiene, and firewall cleanup.
Planning VPC peering
AWS and Azure refuse peering between networks with overlapping CIDRs.
Cleaning route tables
Nested static routes reveal redundant entries.
Removing duplicate firewall objects
Objects contained in broader objects are candidates for removal.
Checking acquisition networks
Networks of an acquired company are checked for overlap before integration.