Cloudformation template to get a dynamically generated CIDR block for subnets based on the selected availability zone

cloudformation-dynamic-cidr.yaml

Mappings:
  Networking:
    AzSuffixToIndex:
      1a: 0
      1b: 1
      1c: 2
      1d: 3
      1e: 4
      1f: 5
      1g: 6
      2a: 0
      2b: 1
      2c: 2
      2d: 3
      2e: 4
      2f: 5
      2g: 6
      3a: 0
      3b: 1
      3c: 2
      3d: 3
      3e: 4
      3f: 5
      3g: 6
      4a: 0
      4b: 1
      4c: 2
      4d: 3
      4e: 4
      4f: 5
      4g: 6

Resources:
  VPC:
    Type: AWS::EC2::VPC
    Properties:
      CidrBlock: 10.0.0.0/16
      EnableDnsSupport: true
      EnableDnsHostnames: true

  PublicSubnet:
    Type: AWS::EC2::Subnet
    Properties:
      VpcId:
        Ref: VPC
      AvailabilityZone:
        Ref: AvailabilityZone
      # Dynamically generate a CIDR block with non-overlapping IP ranges for each possible AZ in the region
      # https://docs.aws.amazon.com/fr_fr/AWSCloudFormation/latest/UserGuide/intrinsic-function-reference-cidr.html
      CidrBlock: !Select [
        !FindInMap [Networking, AzSuffixToIndex, !Select [ 2, !Split [ "-", !Ref AvailabilityZone]]],
        !Cidr [!GetAtt [VPC, CidrBlock], 16, 12]
      ]