Is Fn::Cidr random?

Question

Will Fn::Cidr always output the same values if called with the same input? I want to obtain 3 sub addresses for three different subnets from the same Vpc ip block, can I guarantee that I'll get different addresses for those 3 subnets if I use them as below:

I'm changing Fn::Select's first argument for each one.

"SubBlock1" : { "Fn::Select" : [ 0, { "Fn::Cidr" : ["10.0.1.0/16", 3, 24]} ] }

"SubBlock2" : { "Fn::Select" : [ 1, { "Fn::Cidr" : ["10.0.1.0/16", 3, 24]} ] }

"SubBlock3" : { "Fn::Select" : [ 2, { "Fn::Cidr" : ["10.0.1.0/16", 3, 24]} ] }

Answer 1

To test this, I created a stack with this template:

AWSTemplateFormatVersion: 2010-09-09

Resources:

  Bucket:
    Type: AWS::S3::Bucket

Outputs:
  
  Block1:
    Value: !Select 
        - 0
        - Fn::Cidr: 
          - 10.0.0.0/16
          - 3
          - 8

  Block2:
    Value: !Select 
        - 1
        - Fn::Cidr: 
          - 10.0.0.0/16
          - 3
          - 8

  Block3:
    Value: !Select 
        - 2
        - Fn::Cidr: 
          - 10.0.0.0/16
          - 3
          - 8

(The bucket is included simply because at least one resource is required.)

The output was:

• Block1: 10.1.0.0/24
• Block2: 10.1.1.0/24
• Block3: 10.1.2.0/24

So, yes, it will output the same blocks each time because it is simply dividing the given CIDR as requested.

Some notes:

• I had to supply cidrBits=8 to generate a /24 block
• 10.0.1.0/16 in your example is not a valid /16 block, so I changed it to 10.1.0.0/16