Use IPAM with provided CIDR block to create VPC

[nhhai]

Is your request related to a new offering from AWS?

Yes, can do with the current capabilities.

Is your request related to a problem? Please describe.

In my current organization, people use convention to manage the CIDR blocks for different env / subnets.
E.g: For x.y.0.0/16 VPCs, use y < 100 for prod and stage, use y from 100-199 for dev...

We have existing VPCs and now we're refactoring to use IPAM. We want to keep the current CIDRs block while migrating. It's currently impossible because IPAM only works with netmask_length.

The example here: https://github.com/terraform-aws-modules/terraform-aws-vpc/blob/master/examples/ipam/main.tf#L46 is incorrect, it doesn't work because in the main.tf file, cidr and ipam are exclusive: https://github.com/terraform-aws-modules/terraform-aws-vpc/blob/master/main.tf#L31

Describe the solution you'd like.

I think IPAM should be possible to use with provided CIDRs, as many of us are migrating to it, and also want to avoid future overlapping. It's also easier in case of adding / removing subnets.

Though I think this is more on AWS side that they should support them. As a workaround for now, I think when both ipv4_ipam_pool_id and cidr are provided, the VPC should be created without using IPAM, then create an vpc_ipam_pool_cidr_allocation resource to mark that block in the pool.

I haven't tried this, so this is simply a theory.

Describe alternatives you've considered.

Well, wait for AWS to support this in their VPC creation of course.

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