SubnetExtA:
  Type: AWS::EC2::Subnet
  DependsOn: VpcCidrBlockIpv6
  Properties:
    CidrBlock:
      !Select: [0, !Cidr [ !Ref "VpcCidr", "256", "8" ]] 
    Ipv6CidrBlock:
      !Select: [0, !Cidr [ !Select [ 0, !GetAtt ["VPC", "Ipv6CiderBlocks"], "256", "64" ]]]
    AssignIpv6AddressOnCreation: "true"
    AvailabilityZone: !Join [ "", [ !Ref "AWS::Region", "a" ]]
    Tags: 
      -
        Key: Name
        Value: !Join [ "-", [ !Ref "AWS::StackName", "Ext" ]]
    VpcId: !Ref VPC