Subnet Overlap Checker

What is a subnet overlap?

A subnet overlap is when two networks share one or more addresses. Because valid CIDR blocks are always aligned to power-of-two boundaries, overlap between two of them is much more constrained than it might sound: it's never partial. This tool checks two networks (or an entire pasted list) for exactly this kind of relationship.

How to use the Subnet Overlap Checker

Use "Compare two networks" to check a single pair, or "Analyze network list" to paste an entire address plan (one CIDR per line, with an optional label) and find every duplicate, containment, and adjacency relationship in it at once.

CIDR containment vs. overlap

This is the key fact the whole tool is built around: for two valid, correctly aligned CIDR prefixes of the same address family, if they intersect at all, one of three things is true — they're identical, one fully contains the other, or (trivially) they're the same network. A genuine partial overlap, where each network has some addresses the other doesn't, is not possible between two valid CIDR blocks. Adjacent blocks can touch at their boundaries without overlapping at all.

Why overlapping networks cause problems

Overlapping private address space is a common source of real incidents: VPN tunnels that can't route correctly when both sides use the same range, company mergers where two networks both used 10.0.0.0/8, cloud VPC/VNet peering that silently fails or misroutes, and NAT rules that behave unpredictably when source and destination ranges collide. None of this means overlap is always wrong — a subnet is supposed to be contained inside its parent block — but it's worth confirming deliberately rather than discovering it during a migration.

Duplicate networks

The same CIDR entered, configured, or documented more than once. In a pasted list, this tool aggregates duplicate occurrences into one finding (10.0.0.0/24 — 3 occurrences) rather than reporting every pairwise combination separately.

Adjacent networks

Two networks are adjacent when one's last address is immediately followed by the other's first address, with no overlap. 10.0.0.0/25 and 10.0.0.128/25 are adjacent: the first ends at 10.0.0.127, the second begins at 10.0.0.128.

When adjacent networks can be merged

Adjacency alone doesn't guarantee two networks can be combined into one shorter prefix. They need to be exact aligned siblings: the same size, and positioned so their combined range starts on a boundary valid for the next prefix length up. 10.0.0.0/25 and 10.0.0.128/25 qualify and merge exactly into 10.0.0.0/24. Two /26s that happen to be adjacent but don't share a common /25 parent do not qualify, and this tool won't suggest merging them.

IPv4 vs. IPv6 overlap checking

The same containment/duplicate/adjacency logic applies to IPv6, using 128-bit BigInt arithmetic throughout so precision is never lost, even for enormous ranges. IPv6 has no broadcast address, so IPv6 results never include one.

How to check multiple networks for conflicts

Paste your address plan into Analyze Network List mode, one CIDR per line. The tool sorts and sweeps the list in O(n log n) time rather than comparing every possible pair, which keeps it fast even for large plans, and reports duplicates, containment, and adjacency together in one summary, a conflict matrix, and a findings table.

Subnet Overlap Checker vs. CIDR Range Converter

The CIDR & IP Range Converter converts between CIDR and address-range representations, and can merge a list of CIDRs into the smallest exact set. This tool instead answers a different question about a set of networks: how do they relate to each other, and do any of them conflict?

Subnet Overlap Checker vs. Subnet Calculator

The Subnet Calculator analyzes one network in detail. This tool compares networks against each other. Once you've identified a network of interest here, open it directly in the Subnet Calculator for its full detail.