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aws-cidr-finder master PyPI codecov

  1. Overview
    1. An Example
  2. Installation
  3. Configuration
  4. Contributing


aws-cidr-finder is a Python CLI tool which finds unused CIDR blocks (IPv4 only currently) in your AWS VPCs and outputs them to STDOUT. It is very simple, but can be quite useful for users who manage many subnets across one or more VPCs.

Use aws-cidr-finder -h to see command options.

An Example

It is easiest to see the value of this tool through an example. Pretend that we have the following VPC setup in AWS:

  • A VPC whose CIDR is, with a Name tag of Hello World
  • Six subnets in that VPC whose CIDRs are:
  • An AWS CLI profile named myprofile

aws-cidr-finder allows you to quickly compute the CIDRs that you still have available in the VPC without having to do a lot of annoying/tedious octet math. If we issue this command:

aws-cidr-finder --profile myprofile

We should see this output:

Here are the available CIDR blocks in the 'Hello World' VPC:
CIDR               IP Count
---------------  ----------         8192       32768
Total                 40960

You should notice that by default, aws-cidr-finder will automatically "simplify" the CIDRs by merging adjacent free CIDR blocks so that the resulting table shows the maximum contiguous space per CIDR (in other words, the resulting table has the fewest number of rows possible). This is why the result of the command displayed only two CIDRs: a /19 and a /17.

Note that the first CIDR is /19 instead of, for example, /18, because the /18 CIDR would mathematically have to begin at IP address, and that IP address is already taken by a subnet!

However, we can change this "simplification" behavior by specifying the --prefix CLI flag:

aws-cidr-finder --profile myprofile --prefix 20

Now, the expected output should look something like this:

Here are the available CIDR blocks in the 'Hello World' VPC:
CIDR               IP Count
---------------  ----------         4096        4096        4096        4096        4096        4096        4096        4096        4096        4096
Total                 40960

With the --prefix argument, we can now query our available network space to our desired level of detail. Note that if we specify a --prefix with a value lower than any of the prefixes in the originally-returned list, those CIDRs will be skipped. For example, if we run the following:

aws-cidr-finder --profile myprofile --prefix 18

We should see this output:

Note: skipping 1 CIDR because its prefix is larger than the requested prefix (18).

Here are the available CIDR blocks in the 'Hello World' VPC:
CIDR               IP Count
---------------  ----------       16384       16384
Total                 32768

The CIDR that was skipped was the CIDR because it is impossible to convert a /19 CIDR into one or more /18 CIDRs.


If you have Python >=3.10 and <4.0 installed, aws-cidr-finder can be installed from PyPI using something like

pip install aws-cidr-finder


All that needs to be configured in order to use this CLI is an AWS CLI profile or a keypair. The former may be specified using the --profile argument on the CLI, while the keypair must be specified in environment variables. If both are available simultaneously, aws-cidr-finder will prefer the profile.

The environment variables for the keypair approach are AWS_ACCESS_KEY_ID and AWS_SECRET_ACCESS_KEY (the same values Boto uses).

You should also ensure that the profile/keypair you are using has the AWS IAM access needed to make the underlying API calls via Boto. Here is a minimal IAM policy document that fills this requirement:

  "Effect": "Allow",
  "Action": [
  "Resource": "*"

Read more about the actions shown above here.


See for developer-oriented information.


A Python CLI tool for finding unused CIDR blocks in AWS VPCs.