Subnet Overlap
Paste a list of CIDR networks (IPv4 or IPv6, one per line) and find out which pairs overlap or whether one network contains another, and check whether a single IP falls inside any of them. Nothing is sent to any server.
IP membership
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Conflicting pairs
0| Network A | Relation | Network B | Detail |
|---|
No pair overlaps or contains another: the networks are disjoint.
Range of each network
| Network | Family | Start | End | Addresses | Status |
|---|
How it is calculated · overlap and containment
1. Each CIDR is turned into an integer range
[start, end]. IPv4 uses a 32-bit unsigned
integer; IPv6 a 128-bit one (with BigInt).
2. start = ip & mask and
end = start + 2^(bits − prefix) − 1, where
bits is 32 (IPv4) or 128 (IPv6).
3. Two networks overlap if
start₁ ≤ end₂ && start₂ ≤ end₁.
4. There is containment (one ⊆
the other) if start_a ≤ start_b && end_b ≤ end_a.
Only networks of the same family are compared (IPv4 with IPv4, IPv6 with IPv6).
Runs locally in your browser · no sign-up · nothing leaves your browser.
How it works
The checker takes a list of CIDR networks (IPv4 or IPv6, one per line) and detects which pairs overlap and which ones are contained inside another network. You can also enter a single IP and the tool flags every network on the list that contains it. It is the typical check before advertising routes, building site-to-site VPNs or assigning blocks to new sites.
Each CIDR is converted to an integer range [start, end]: IPv4 as a 32-bit unsigned integer and IPv6 as a 128-bit integer (using BigInt). The start is ip AND mask and the end is start + 2^(bits − prefix) − 1. Two networks overlap if start₁ ≤ end₂ and start₂ ≤ end₁; there is containment when one range fits completely inside the other. Only networks of the same family are compared (IPv4 with IPv4, IPv6 with IPv6).
Example: do 192.168.0.0/24 and 192.168.0.128/25 conflict?
192.168.0.0/24covers the range .0 to .255 (256 addresses).192.168.0.128/25covers the range .128 to .255 (128 addresses).- Since .128 ≤ .255 and .0 ≤ .255, the ranges intersect; and because the /25 fits entirely inside the /24, the verdict is containment: the /24 contains the /25.
- The IP
192.168.0.130falls inside both networks, so the tool flags them both.