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IP Range To CIDR Calculator

Work out ip range to cidr instantly with clear inputs, formula shown and shareable results.

About IP Range To CIDR Calculator

An arbitrary address range rarely maps to one CIDR prefix, because every prefix must start on a boundary that is a multiple of its own size. The greedy algorithm takes the largest aligned block that fits at the current address, emits it, and repeats until the end of the range is reached — which is exactly how firewall and ACL generators expand a range.

Formula

Greedy CIDR decomposition

at each address take the largest 2^k block where address mod 2^k = 0 and address + 2^k - 1 <= end; repeat

Worked example

With the default values below, IP Range To CIDR Calculator returns CIDR blocks covering the range of 192.168.1.10/31, 192.168.1.12/30, 192.168.1.16/28, 192.168.1.32/27, 192.168.1.64/26, 192.168.1.128/26, 192.168.1.192/29, 192.168.1.200/32. Change any input above and every figure here is recalculated in your browser.

Inputs used

Start of range
192.168.1.10
End of range
192.168.1.200

Results

CIDR blocks covering the range
192.168.1.10/31, 192.168.1.12/30, 192.168.1.16/28, 192.168.1.32/27, 192.168.1.64/26, 192.168.1.128/26, 192.168.1.192/29, 192.168.1.200/32
Number of blocks required
8
Addresses in range
191
Largest single block
/26

Step by step

  1. 1Values used
  2. 2Greedy CIDR decomposition
  3. 3CIDR blocks covering the range= 192.168.1.10/31, 192.168.1.12/30, 192.168.1.16/28, 192.168.1.32/27, 192.168.1.64/26, 192.168.1.128/26, 192.168.1.192/29, 192.168.1.200/32
  4. 4Number of blocks required= 8
  5. 5Addresses in range= 191
  6. 6Largest single block= /26

Assumptions and limits

  • The result is derived only from the 2 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
  • Figures are rounded for display to whole numbers, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

Why does a small range need several prefixes?

Alignment. 192.168.1.10 is not a multiple of 8, so the first block can only be a /31 or /32 until the pointer reaches an aligned boundary.

How do I get a single prefix?

Choose a start address that is a multiple of the block size and an end address one below the next boundary — for example 192.168.1.0 to 192.168.1.255 is exactly one /24.

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