Why do I get several prefixes instead of one?

A prefix must start on a multiple of its own size. A range that does not begin and end on such a boundary cannot be expressed as one block, so it is split into the fewest that fit exactly.

Will the result ever cover addresses outside my range?

No. That is the point of the exact conversion. If you want a single covering supernet instead, that is a different question and a different answer, since it would include addresses you did not ask for.

Yes, using the same algorithm. Ranges within IPv6 can produce large prefix counts if the boundaries are awkward.

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IP Range to CIDR

Give it a start and an end address. It returns the smallest exact set of prefixes covering that range and nothing more.

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Start address

End address

Why a range often needs several prefixes

A CIDR prefix can only start on an address that is a multiple of its own size. So a range beginning at 192.168.1.1 cannot be a single block — the first address is not aligned to anything larger than a /32.

The algorithm walks from the start, taking the largest block that both begins at the current address and does not run past the end, then repeats. That produces the minimal exact set.

Exact means exact. The result never includes an address outside the range you gave. Tools that return a single "closest" prefix are giving you a supernet that covers addresses you did not ask for, which in a firewall rule is a security problem rather than a rounding convenience.

Related tools

CIDR Aggregator merges a list of prefixes. CIDR Range Calculator goes from a prefix to its boundaries.