Overlapping CIDR Detector

CIDR overlap is one of those things that only surfaces after it has caused damage. A firewall rule for 10.0.0.0/8 swallows the exception someone wrote for 10.0.5.0/24. Two VPC subnets with crossing ranges make peering impossible. A more specific static route captures traffic that was supposed to take another path. In a list a few dozen lines long, nobody catches this by eye.

Paste the list of IPv4 blocks, one per line, and the page compares every pair. Each block is turned into the numeric range it covers — first and last address — so the comparison becomes a question of ranges that cross. The result separates the cases: containment, when one block sits entirely inside another; duplication, when both cover exactly the same range; and partial overlap, when they cross without either containing the other.

The address you supply is reduced to the start of its network before the maths, so 10.0.0.37/24 is treated as 10.0.0.0/24 — the same thing a router does. Only IPv4 goes in here, and the tool points at conflicts without trying to consolidate anything: aggregating ranges is a design decision, not an arithmetic one, and it depends on where each route points.