CIDR Merge Calculator
Merge consecutive CIDR blocks into a single aggregate and check that the starting network is correctly aligned.
About CIDR Merge Calculator
Merging 2^k adjacent blocks shortens the prefix by k bits. The merge is only lossless if the first network address sits on a boundary of the larger block, otherwise the aggregate covers space that belongs to a neighbour. Aggregation is what keeps routing tables and security-group rule counts small, but a misaligned summary advertises address space you do not own.
Formula
CIDR Merge
merged prefix = p − ceil(log2(blocks)); the aggregate is valid only when the first network is a multiple of the aggregate block size.
- p
- Prefix length of each component block
- blocks
- Number of consecutive blocks being merged
Worked example
With the default values below, CIDR Merge Calculator returns Merged CIDR of 192.168.8.0/22. Change any input above and every figure here is recalculated in your browser.
Inputs used
- First network address
- 192.168.8.0
- Prefix length of each blockaccepts 1–32 bits
- 24 bits
- Number of consecutive blocksaccepts 1–4,096 blocks
- 4 blocks
Results
- Merged CIDR
- 192.168.8.0/22
- Merged Prefix Length
- 22
- Correctly Aligned
- Yes
- Blocks Absorbed but Unused
- 0
Step by step
- 1Values used
- 2CIDR Merge
- 3Merged CIDR= 192.168.8.0/22
- 4Merged Prefix Length= 22
- 5Correctly Aligned= Yes
- 6Blocks Absorbed but Unused= 0
Assumptions and limits
- The result is derived only from the 3 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
How is CIDR Merge calculated?
merged prefix = p − ceil(log2(blocks)); the aggregate is valid only when the first network is a multiple of the aggregate block size. Merging 2^k adjacent blocks shortens the prefix by k bits. The merge is only lossless if the first network address sits on a boundary of the larger block, otherwise the aggregate covers space that belongs to a neighbour.
Why does CIDR Merge matter?
Aggregation is what keeps routing tables and security-group rule counts small, but a misaligned summary advertises address space you do not own.
What values do I need to enter?
This calculator takes 3 inputs: First network address, Prefix length of each block, Number of consecutive blocks. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.