How to retrieve CIDR ranges

vpc_subnet_cidr.tf

# Fetch the VPC as a Data Source
data "aws_vpc" "vpc" {
  filter {
    name   = "tag:Name"
    values = ["vpc-name"]
  }
}

# Fetch the Subnet IDs from the VPC
data "aws_subnet_ids" "public" {
  vpc_id = "${data.aws_vpc.vpc.id}"
  tags = {
      another-arbitrary-tag = "public"
  }
}

# Fetch the Subnets themselves using their IDs
data "aws_subnet" "public" {
  count = "${length(data.aws_subnet_ids.public.ids)}"
  id    = "${data.aws_subnet_ids.public.ids[count.index]}"
}

# Output the values from the Subnets as necessary
output "public_subnet_cidr_blocks" {
  value = ["${data.aws_subnet.public.*.cidr_block}"]
}