What is a subnet overlap?
Two subnets overlap when they share at least one IP address. In CIDR terms this means one subnet sits completely inside another, for example 10.0.5.0/24 inside 10.0.0.0/16, or the same subnet appears twice.
Overlaps cause real trouble. A router cannot tell which network an address belongs to, two devices can end up with the same IP, and VPN tunnels, VPC peering and site-to-site links fail when both sides use the same address space. It is much cheaper to find an overlap on paper than after the link is built.
How to use the Subnet Overlap Checker
Type or paste the subnets into Enter Subnets, one per line, like 192.168.0.0/24. You can also write the mask in full, like 192.168.0.0/255.255.255.0. Click Copy Example if you want a sample to paste in and try.
The result appears below the form (pressing Enter in the box adds a new line, so use the button). From then on it follows your changes, so you can add or remove a line and watch the result update. If a line is not valid, a red message names it.
A green No overlaps found or a red Overlaps found tag tells you the answer at a glance. Overlapping Subnets lists each clashing pair and the range they share. Subnet Status lists every subnet you entered, and says whether it is clear or how many others it overlaps with.
Tick or untick boxes in Show Columns to choose what you see. The first four boxes are for the overlap table and the last three are for the status table. On a phone or tablet that row slides sideways.
Copy puts both tables on your clipboard and Download saves them as a CSV file. Both use only the columns you ticked. Clear All empties the box and hides the result.
Common slip-ups
- "Enter at least two subnets". One subnet has nothing to be compared with.
- A line is marked as not valid. Each subnet needs a prefix, such as
/24. An address with no prefix is not enough. - A subnet shows as overlapping with a bigger one. That is correct. A
/24inside a/16shares all of its addresses with it.
How overlaps are found
Two address ranges overlap when each one starts before the other one ends:
For CIDR subnets there is a shortcut. A subnet always starts on a boundary that fits its size, so two subnets either do not touch, or one fits entirely inside the other. There is no partial overlap. The checker sorts all subnets by start address and walks down the list once. At each step it knows which bigger subnets still contain the current one, and those are the overlaps. This is fast even for thousands of subnets.
Example
Take these five subnets:
| Subnet | Range | Total IPs | Status |
|---|---|---|---|
| 10.0.0.0/16 | 10.0.0.0 – 10.0.255.255 | 65,536 | Overlaps with 1 subnet |
| 10.0.5.0/24 | 10.0.5.0 – 10.0.5.255 | 256 | Overlaps with 1 subnet |
| 172.16.0.0/24 | 172.16.0.0 – 172.16.0.255 | 256 | No overlap |
| 192.168.1.0/24 | 192.168.1.0 – 192.168.1.255 | 256 | Overlaps with 1 subnet |
| 192.168.1.128/25 | 192.168.1.128 – 192.168.1.255 | 128 | Overlaps with 1 subnet |
There are 2 overlapping pairs:
| Subnet A | Subnet B | Relationship | Overlap Starts | Overlap Ends | Overlapping IPs |
|---|---|---|---|---|---|
| 10.0.0.0/16 | 10.0.5.0/24 | A contains B | 10.0.5.0 | 10.0.5.255 | 256 |
| 192.168.1.0/24 | 192.168.1.128/25 | A contains B | 192.168.1.128 | 192.168.1.255 | 128 |
The overlapping range is always the whole of the smaller subnet, because it sits fully inside the bigger one.
When to use it
Run it before you create a new VPC or VNet, set up a VPN or peering link, add a VLAN, merge two company networks, or add a route to a routing table. It also helps you audit an existing address plan or a spreadsheet of subnets, and catch duplicates that crept in over the years.
To lay out new subnets without clashes, use the VLSM Calculator or the FLSM Calculator. To join several networks into one summary route, use the Supernetting Calculator.