What is the difference between contained and overlapping?

Contained means every address of the inner prefix is inside the outer one. Overlapping means they share some addresses but neither holds the other, which almost always indicates a planning error.

Why is usable sometimes the same as total?

For IPv6 always, because there is no broadcast address. For IPv4 at /31 and /32, where the addresses that would normally be reserved are usable.

Does it work for IPv6?

Yes. Prefix lengths run 0 to 128 and the same containment logic applies.

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CIDR Range Calculator

A prefix in, its boundaries out — first address, last address, how many there are, and what it sits inside.

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Prefix

Is this address or prefix inside it? (optional)

Notes

Total addresses counts every address in the prefix. Usable hosts subtracts the network and broadcast addresses for IPv4 — except at /31 and /32, where RFC 3021 and host routing mean all addresses are usable. IPv6 has no broadcast address, so the two figures are always equal.

The containment check answers the question that actually causes arguments: whether one prefix sits inside another. It distinguishes contained from overlapping, because a partial overlap is a different problem and usually a mistake.

Related tools

Subnet Calculator shows the mask and the bits. IP Range to CIDR goes the other way, from two addresses to prefixes.