What counts as an overlap

Two CIDR blocks overlap whenever their address ranges share even a single address — that includes the obvious case of identical blocks, but also the less obvious case of one block being entirely nested inside another, like 10.0.0.0/25 sitting fully inside 10.0.0.0/24. This tool flags both kinds identically, because both represent the same underlying problem: an address that's supposed to belong to exactly one allocation actually belongs to two, which is exactly the condition that causes routing ambiguity, VPC peering conflicts, and "why is this DHCP scope handing out addresses that are already static-assigned elsewhere" incidents.

Where this catches real mistakes

The most common place this matters in practice is when subnet allocations accumulate over time across a spreadsheet, a wiki page, or several people's memory rather than a single source of truth — a new /24 gets carved out without checking it against everything already allocated, and it isn't until two hosts can't reach each other (or a VPC peering connection silently fails) that the overlap is discovered. Running the full list through a check like this before adding a new block is much cheaper than debugging the resulting routing conflict after the fact.