CIDR Overlap Checker
Check whether two IPv4 CIDR blocks overlap. Parses each block into its network range and reports any conflicting address range — useful for network planning, firewall rules and avoiding routing conflicts.
Input
First IPv4 block in CIDR notation.
Second IPv4 block in CIDR notation.
Output
| Property | Value |
|---|---|
| No data yet | |
Guides
What does "CIDR overlap" mean?
A CIDR block like 192.168.1.0/24 describes a contiguous range of IPv4 addresses — the prefix (/24) says how many leading bits are fixed, leaving the rest as host addresses. Two CIDR blocks overlap when their address ranges share at least one IP address, even partially. For example, 192.168.1.0/24 (which covers 192.168.1.0 through 192.168.1.255) overlaps with 192.168.1.128/25 (which covers 192.168.1.128 through 192.168.1.255), because the second block is entirely contained inside the first.
Why overlap checks matter
- Network planning — when subnetting a larger block, overlapping subnets waste address space and cause ambiguous routing.
- Firewall and ACL rules — two rules targeting overlapping ranges can silently shadow each other, letting traffic through (or blocking it) unexpectedly.
- VPN and cloud peering — connecting two networks (VPC peering, site-to-site VPNs) with overlapping private ranges (e.g. both using
10.0.0.0/8) breaks routing entirely; this has to be caught before the connection is made, not after. - DHCP and static allocations — overlapping pools hand out the same address to two devices, causing IP conflicts.
How to use it
- Enter the first block in CIDR Block A, e.g.
192.168.1.0/24. - Enter the second block in CIDR Block B, e.g.
192.168.1.128/25. - The result table updates automatically, showing each block's resolved address range, whether they overlap, and — if so — the exact overlapping range.
How the check works
Each block is parsed into a network address and prefix length, then expanded into a start and end address by zeroing out the host bits (the address portion outside the prefix). Two ranges overlap exactly when start A ≤ end B and start B ≤ end A. This correctly catches every overlap case: one block fully containing another, a partial overlap at the edges, or two blocks being identical.
FAQ
Does the address I enter need to be the exact network address?
No — if you enter a host address like 192.168.1.5/24, the tool normalizes it to the network address (192.168.1.0/24) before comparing, matching how routers interpret CIDR notation.
What prefix lengths are supported?
Any IPv4 prefix from /0 (the entire address space) to /32 (a single host).
Does this support IPv6?
Not currently — this tool is IPv4-only. Each address must be a standard dotted-quad (four octets, 0–255).
What if I enter an invalid CIDR block? The tool reports a clear error for that block (missing prefix, out-of-range octet, invalid prefix length) instead of a result, so you can fix the input.
Privacy
This check runs entirely in your browser — the CIDR blocks you enter are never sent to a server or stored.
Use it from code
From 3 credits per callREST API
curl -X POST https://api.iotools.cloud/v1/tool/cidr-overlap-checker \
-H "Authorization: Bearer YOUR_API_KEY" \
-H "Content-Type: application/json" \
-d '{
"cidrA": "192.168.1.0/24",
"cidrB": "192.168.1.128/25"
}'Swap in your own key from your account. The tool's fields are the body — no wrapper.
Ask an AI agent
Use the IOTools `cidr-overlap-checker` tool (CIDR Overlap Checker) on this input:
YOUR_INPUT_HEREPaste this at any agent connected to the IOTools MCP server, then add your input.