Convert IP Range to CIDR List

A vendor tells you to allow 10.0.0.1 through 10.0.5.255. Your firewall field, your bucket policy and your security group all want CIDR notation. Doing that translation by hand is tedious and easy to get wrong, and a wrong block either opens addresses you never meant to open or leaves out the exact IP your customer calls the API from.

The algorithm is greedy and walks from the start toward the end. At each step it picks the largest aligned block that begins at the current address without running past the finish, writes that CIDR down and moves on. Alignment drives the length of the list: 10.0.0.1 to 10.0.5.255 produces eleven blocks, starting with a lonely /32. Shift the start to 10.0.0.0 and the same span collapses to two, 10.0.0.0/22 and 10.0.4.0/23.

If the list comes back too long for your rule field, try aligning the edges to powers of two, since one spare address at the boundary often halves the output. The result is exact and never includes an IP outside the range you gave, unlike the old habit of rounding everything up to a /24. A start above the end returns an error, and only IPv4 is accepted. Everything is computed in your browser.