CIDR Overlap Checker | Find Conflicting Subnets

CIDR overlap checker that finds conflicting subnets in a list of IPv4 and IPv6 blocks. Spot VPC peering clashes and duplicate ranges before you deploy. Free. Designed specifically for developers and professionals, this free online networking runs entirely in your browser to process tasks quickly and securely. Enjoy unlimited usage with no data tracking and no registration required.

Also known as: cidr overlap checker · check if cidr blocks overlap · subnet overlap checker · vpc cidr overlap · vpc peering cidr conflict · cidr conflict detector · ip range overlap calculator · ipv6 cidr overlap

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devops and infrastructure tools

networking

CIDR Block List

Overlap Report

Overlapping blocks will appear here

About CIDR Overlap Checker

Paste a list of IPv4 and IPv6 CIDR blocks and find out which ones collide. A CIDR overlap checker is the quick sanity test before you peer two VPCs, connect a VPN to an office network, or hand out the next subnet in a shared address plan.

Key Features

Finds every pair where one block contains another or two blocks are identical, for IPv4 and IPv6 in the same list
Shows how many addresses each conflict shares, so a stray /32 is easy to tell apart from a duplicated /16
Accepts an optional label on each line, such as prod-vpc or office-nyc, and keeps it in the report
Flags blocks written with host bits set, like 192.168.50.7/24, and checks them as the real network 192.168.50.0/24
Skips comments after # or // and reports any line it cannot parse instead of failing the whole run

Use Cases

Confirm two AWS VPCs or Azure VNets have disjoint CIDR ranges before creating a peering connection
Check that a site-to-site VPN range does not collide with the 10.0.0.0/8 space already used in a data center
Audit a Terraform or CloudFormation variable file for duplicate subnet allocations across regions such as us-east-1 and us-west-2
Review a Kubernetes pod and service CIDR plan against the node network before a cluster is created
Validate a firewall allowlist for entries that are already covered by a broader range

Supported Formats

IPv4 CIDR

IPv6 CIDR

Bare IP addresses (/32 and /128)

Plain text lists

Related Keywords

cidr overlap checker

check if cidr blocks overlap

subnet overlap checker

vpc cidr overlap

vpc peering cidr conflict

cidr conflict detector

ip range overlap calculator

ipv6 cidr overlap

Why do VPC peering connections fail on overlapping CIDR blocks?

A route table needs one unambiguous next hop per destination. If two peered networks both claim 10.0.0.0/16, a packet for 10.0.4.9 could belong to either side, so AWS refuses the peering request and most VPN gateways refuse the route. CIDR blocks never partially overlap: two blocks are either completely disjoint or one sits entirely inside the other. That is why this tool reports containment and duplicates, and why one stray match is enough to block a connection.

Privacy & Security

All processing happens locally in your browser. Your data never leaves your computer, ensuring complete privacy and security.

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Frequently asked questions

What is CIDR Overlap Checker used for?

CIDR overlap checker that finds conflicting subnets in a list of IPv4 and IPv6 blocks. Spot VPC peering clashes and duplicate ranges before you deploy. Free.

Is CIDR Overlap Checker free to use?

Yes. CIDR Overlap Checker is completely free with no usage limits, and there is no paid tier required to use it.

Do I need an account to use CIDR Overlap Checker?

No. CIDR Overlap Checker works immediately in your browser with no signup, login, or email address required.

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