AWS VPCs: Calculating Subnets in CloudFormation

Virtual Private Cloud is a construct in AWS that gives the customer their own, er, virtual network for the deployment of network based resources such as virtual machines and more. Its been around for nearly a decade, and is a basic construct that helps provide security of those resources within an AWS Region.

CloudFormation is the (text, either YAML or JSON) templating language (service) that can take a definition of resources you would like configured, and does the execution of creating these resources for you, saving you the hassle of having to either navigate the web console for hours, or scripting up many API calls (which could be thousands of API create calls).

VPCs can be quite complex; they can specify subnets for resources, across multiple Availability Zones within a Region, define routing tables, Endpoints to create, and much more. So it probably comes as no surprise that managing a VPC via CloudFormation is a natural desire. The configuration of the virtual network for a workload needs to be as management in a CI/CD fashion as the workload that will live in there.

But there’s often been a limitation in making this simple; mathematics.
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