IP range to CIDR converter

Turn a first and last IPv4 address into the smallest list of CIDR blocks.

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A clear answer. Here’s how.

Starting at the first address, repeatedly takes the largest block that is aligned to its own size and still fits before the last address. The result is the minimal exact cover: the blocks add up to the range with no extra addresses. A range such as 192.168.0.0 to 192.168.1.255 becomes one /23, while an unaligned range needs several blocks.

See a worked example

Worked example

A fixed demonstration, separate from the values you enter above.

First address: 10.0.0.5
Last address: 10.0.0.20

Example result

Example result
CIDR block	First address	Last address	Addresses
10.0.0.5/32	10.0.0.5	10.0.0.5	1
10.0.0.6/31	10.0.0.6	10.0.0.7	2
10.0.0.8/29	10.0.0.8	10.0.0.15	8
10.0.0.16/30	10.0.0.16	10.0.0.19	4
10.0.0.20/32	10.0.0.20	10.0.0.20	1

5 blocks cover 16 addresses exactly: the smallest CIDR list that spans the range with no extra addresses.