CIDR Merge Calculator
Merge consecutive CIDR blocks into a single aggregate and check that the starting network is correctly aligned.
Inputs
Merged CIDR
192.168.8.0/22
Merged Prefix Length
22
Correctly Aligned
Yes
Blocks Absorbed but Unused
0
Step by step
Values used
First network address = 192.168.8.0; Prefix length of each block = 24 bits; Number of consecutive blocks = 4 blocks
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.
Merged CIDR
= 192.168.8.0/22
Merged Prefix Length
= 22
Correctly Aligned
= Yes
Blocks Absorbed but Unused
= 0
How it works
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
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.