IP range to CIDR converter
Firewalls and allow lists usually want CIDR blocks; this splits any range into the exact blocks that cover it with nothing extra.
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Processing and privacy: The result is calculated inside your browser and your data is not sent to any server.
How to use it
- Enter start address and end address.
- The result appears straight away and updates as you edit.
- Copy the result, or copy a link that keeps the same values to share it.
How it is calculated
From the start address take the largest block of 2^k addresses that is aligned (start divisible by 2^k) and does not pass the end, then move on and repeat. Prefix = 32 minus k.
Example
Inputs: Start address = 10.0.0.1, End address = 10.0.0.10. Result: CIDR blocks: 10.0.0.1/32 10.0.0.2/31 10.0.0.4/30 10.0.0.8/31 10.0.0.10/32; Number of blocks: 5; Addresses covered: 10.
10.0.0.1
10.0.0.1/32 10.0.0.2/31 10.0.0.4/30 10.0.0.8/31 10.0.0.10/32
Limits and caveats
- IPv4 only.
- Unaligned ranges produce several blocks; that is the minimum possible, not an error.
Questions about this tool
Why is the result not always one block?
A CIDR block must start on a boundary divisible by its size, so a range that starts or ends mid-block needs several.
Do the blocks cover addresses outside my range?
No. They cover exactly the range from start to end.
What is the most blocks a range can need?
Any IPv4 range needs at most 62 blocks.