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.

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 172.31.0.0/16, with a Name tag of Hello World
• Six subnets in that VPC whose CIDRs are:
  • 172.31.0.0/20
  • 172.31.16.0/20
  • 172.31.32.0/20
  • 172.31.48.0/20
  • 172.31.64.0/20
  • 172.31.80.0/20

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
---------------  ----------
172.31.96.0/19         8192
172.31.128.0/17       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 172.31.64.0, 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
---------------  ----------
172.31.96.0/20         4096
172.31.112.0/20        4096
172.31.128.0/20        4096
172.31.144.0/20        4096
172.31.160.0/20        4096
172.31.176.0/20        4096
172.31.192.0/20        4096
172.31.208.0/20        4096
172.31.224.0/20        4096
172.31.240.0/20        4096
Total                 40960

With the --prefix argument, we can now query our available network space to our desired level of detail, as long as we do not specify a smaller prefix than the largest prefix in the original list. For example:

$ aws-cidr-finder --profile myprofile --prefix 18
Desired prefix (18) is incompatible with the available CIDR blocks!
Encountered a CIDR whose prefix is 19, which is higher than 18. Offending CIDR: 172.31.96.0/19
Run the command again without the --prefix argument to see the full list.

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 profile or 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": [
    "ec2:DescribeVpcs",
    "ec2:DescribeSubnets"
  ],
  "Resource": "*"
}

Read more about the actions shown above here.

See CONTRIBUTING.md for developer-oriented information.