A complete guide to how the CIDR Overlap Checker detects subnet conflicts, the binary math behind CIDR notation, and why overlapping ranges cause real network problems.
Launch the CIDR Overlap Checker →The CIDR Overlap Checker takes two IP subnet ranges, each expressed in CIDR notation (such as 10.0.1.0/24), and determines whether they overlap — meaning whether any individual IP address would fall inside both ranges simultaneously. This is a deceptively simple-sounding question that comes up constantly in real network design and troubleshooting work, where overlapping address ranges cause genuinely confusing, hard-to-diagnose connectivity problems.
The tool reports a clear yes-or-no overlap verdict, along with the specific overlapping address range when one exists, so you can see exactly which addresses are in conflict rather than just being told that a conflict exists somewhere.
Every device on an IP network relies on routing tables to decide where to send traffic for a given destination address. When two separate network segments are configured with overlapping address ranges — for example, if a company's main office uses 10.0.1.0/24 and a newly acquired branch office, set up independently, also happens to use 10.0.1.0/24 — a router or VPN gateway trying to connect those two networks together has no way to correctly determine which network a packet destined for an address in that range should actually go to, because the same address exists in both places.
This exact scenario happens surprisingly often in real organizations: multiple sites, each configured independently (sometimes by different IT teams, sometimes years apart, sometimes using the same default address range recommended in a vendor's quick-start guide), later need to be connected via a VPN or merged during a company acquisition, and only then does anyone discover the address ranges collide. The resulting symptoms are often confusing precisely because they look like a general connectivity failure rather than an obvious configuration mistake — some traffic might work, some might silently vanish, and standard troubleshooting steps often don't immediately point to the real cause.
Checking for overlap before connecting two networks, rather than after symptoms appear, is a standard and important step in any network merger, VPN site-to-site setup, or cloud virtual network peering configuration, which is exactly the use case this tool is built for.
A CIDR range like 10.0.1.0/24 represents a contiguous block of IP addresses, defined by a starting address (the network address) and a prefix length (the /24 part) that determines how many addresses the block contains. Converting that notation into a precise numeric range requires calculating the subnet mask from the prefix length, applying that mask to find the exact network (starting) address, and then calculating the exact broadcast (ending) address by combining the network address with the inverted mask.
Once both input ranges have been converted into their precise starting and ending numeric address boundaries, determining overlap becomes a straightforward numeric comparison: two ranges overlap if and only if the start of one range falls at or before the end of the other range, and the end of that same range falls at or after the start of the other — the standard interval-overlap test used throughout computer science for any pair of numeric ranges, applied here to IP addresses treated as plain 32-bit integers.
This same underlying integer-based IP arithmetic (converting dotted-decimal notation to a 32-bit integer and back, and applying bitwise mask operations) is shared across several tools on this site, including the IP Subnet Calculator and the VLAN/Subnet Planner, since it's the same fundamental mathematics regardless of which specific question is being asked of it.
CIDR (Classless Inter-Domain Routing) notation combines an IP address with a slash and a number, like /24, that specifies how many of the address's 32 bits are fixed as the "network" portion, with the remaining bits available for individual host addresses within that network. A smaller number after the slash means fewer bits are fixed for the network, which means more bits are available for hosts, which means a larger overall range of addresses.
A /24 network fixes the first 24 bits (the first three octets in dotted-decimal notation) and leaves the last 8 bits free, producing exactly 256 possible addresses (2 raised to the power of 8). A /25 network fixes one additional bit, leaving only 7 bits free, producing exactly 128 addresses — half the size of a /24. Each additional bit added to the prefix length cuts the available address range in half; each bit removed doubles it.
This halving-and-doubling relationship is why two CIDR ranges can overlap in non-obvious ways: a /25 network is always either the first half or the second half of some corresponding /24 network, a /26 is always one quarter of a /24, and so on. Visualizing CIDR ranges as nested binary subdivisions of address space, rather than as arbitrary number ranges, makes it much easier to understand at a glance why certain combinations of ranges are guaranteed to overlap and others are guaranteed not to.
A /24 network starting at 192.168.1.0 covers every address from 192.168.1.0 through 192.168.1.255. Splitting it into two /25 networks produces exactly 192.168.1.0/25 (covering 192.168.1.0 through 192.168.1.127) and 192.168.1.128/25 (covering 192.168.1.128 through 192.168.1.255). These two /25 halves do not overlap each other, but both are fully contained within, and therefore both overlap, the original /24.
192.168.1.0/24.192.168.1.128/25.Checking 10.0.0.0/24 against 10.0.0.128/25 would correctly report an overlap, since the second range (addresses 10.0.0.128 through 10.0.0.255) is entirely contained within the first range (addresses 10.0.0.0 through 10.0.0.255). This is a case of one subnet being fully nested inside a larger one, which still counts as overlapping even though neither range's boundary partially crosses the other's — any shared address at all is sufficient to constitute an overlap.
All overlap calculations happen entirely within your browser using plain JavaScript arithmetic. No network request of any kind is made by this tool, and no data you enter — the CIDR ranges you check — is ever transmitted anywhere, logged, or stored. This is one of the simplest tools on the site from a privacy standpoint, since it has no dependency on any third-party service whatsoever.
This still counts as an overlap. Any shared address between the two ranges, whether it's a partial boundary overlap or one range being entirely nested inside the other, means the two ranges cannot be used independently without conflict.
This tool is built specifically for IPv4 CIDR notation. IPv6 uses a much larger address space (128 bits instead of 32) and different practical conventions around subnetting, which would require separate dedicated handling.
If both sides of a VPN connection use overlapping address ranges, routers on either end cannot unambiguously determine whether a packet destined for an address in the shared range belongs to the local network or the remote network, which breaks routing in ways that are often confusing to diagnose after the fact.
Yes. Overlap is defined purely by whether any individual IP address exists in both ranges, regardless of how differently sized the two ranges are or whether one is fully contained within the other.