---
AWSTemplateFormatVersion: "2010-09-09"
Parameters:
  Environment:
    Type: String
Mappings:
  Environment:
    acceptance:
      Subnets: "AcceptanceSubnets"
    testing:
      Subnets: "TestingSubnets"
    production:
      Subnets: "ProductionSubnets"
  AcceptanceSubnets:
    eu-west-1a:
      Management: "XXX.XXX.XXX.XXX/XX"
Resources:
  mySubnet:
    Type: AWS::EC2::Subnet
    Properties:
      VpcId: vpc-123456
      # nested and standard checks
      CidrBlock: !FindInMap [!FindInMap ["Environment", !Ref "Environment", "Subnets"], "eu-west-1a", "Management"]