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vpc-Xazs: Smaller subnet size options #180

@bedge

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@bedge

Re: vpc/vpc-*azs.yaml

Feature suggestion/guidelines for possible patch.

Class B subnet granularity is a lot for a single VPC. If one wants to preserve the option of VPC peering, then using less than a class B may be preferable.

Rather than basing everything off of one param:

CidrBlock: !Sub '10.${ClassB}.0.0/16'

Ideally one could add new params with defaults that provide the same behavior unless specified. However given that arithmetic expressions aren't supported, this implies some preprocessing, or a breaking change that uses different params all together.

The minimum number of params one would need are, I think:

IpBase, 10.x.x.x
PrivateSubnetMask: 16
PublicSubnetMask: 20

To continue with that line of reasoning for a bit, one could do this with some pre-processing of the template, despite the lack of finesse. The custom resource thing just feels way to heavy handed for simple arithmetic (http://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/template-custom-resources.html)
Some level of validation to prevent illegal combinations wouldn't be bad either.

Is there an accepted templating format for yaml?

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