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IP Range to CIDR
Convert a start and end address into the smallest set of CIDR blocks.
Quick Tool Overview
The IP Range to CIDR converter turns an arbitrary start and end address into the smallest set of CIDR blocks that covers it exactly — no gap, no overlap — which is what a firewall rule actually needs.
- What it does
- Converts an address range into CIDR blocks
- You give it
- A start and an end address
- You get back
- The smallest exact set of blocks
- Best for
- Firewall rules, security groups and allowlists
About the IP Range to CIDR
Firewalls, security groups and allowlists take CIDR blocks, and the range you actually need is almost never a single tidy one. Give a start and end address and this produces the smallest set of blocks covering exactly that range — no gap, no overlap.
A range only collapses to one block when it starts on a power-of-two boundary and has a power-of-two length. Anything else needs several, and the algorithm is greedy: at each step it takes the largest block that both begins at the current address and fits inside what is left. The total is shown against the range size so you can confirm the coverage is exact, which is the thing that matters when the output becomes a firewall rule.
Key features of the IP Range to CIDR
Provably minimal
Greedily taking the largest aligned block that fits is optimal for this problem, so no shorter list covers the same range.
Exact coverage
Every block sits entirely inside the range, so the rule never allows an address you did not intend.
Coverage verified
The total of the blocks is compared against the range size, so a mistake would be visible rather than silent.
Explains the alignment
Why a range needs several blocks — CIDR requires a power-of-two size starting on a power-of-two boundary.
Class boundaries ignored
As CIDR does; a range crossing from 10.x into 11.x is decomposed like any other.
How to use the IP Range to CIDR
-
1
Enter the start address
The first address the range should include.
-
2
Enter the end address
The last one, inclusive.
-
3
Read the blocks
The smallest set that covers the range exactly.
-
4
Check the coverage
The total is compared against the range size, so an error is visible.
Frequently asked questions
Because CIDR blocks must start on a power-of-two boundary and have a power-of-two size. A range from .1 to .254 satisfies neither, so it decomposes into a series of aligned blocks.
Yes. Greedily taking the largest aligned block that fits is provably optimal for this problem, so no shorter list of CIDR blocks covers the same range.
No. Every block is chosen to fit entirely inside the range, so the coverage is exact — which is essential when the output becomes a firewall rule.
It works fine. CIDR ignores the old class system entirely; a range from 10.255.255.250 to 11.0.0.5 simply produces the blocks that cover it.
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