subnet math

main.tf

provider "aws" {
  region = "us-west-2"
}

variable "az_number" {
  default = {
    a = 1
    b = 2
    c = 3
    d = 4
    e = 5
    f = 6
  }
}

variable "public_netnum_offset" {
  default = 0
}

variable "private_netnum_offset" {
  default = 100
}

variable "vpc_cidr" {
  default = "10.0.0.0/16"
}

variable "newbits" {
  default = "8"
}

locals {
  az = ["us-west-2a", "us-west-2b", "us-west-2c"]
}

data "aws_availability_zone" "az" {
  count = "${length(local.az)}"
  name  = "${local.az[count.index]}"
}

resource "null_resource" "subnets" {
  count = "${length(local.az)}"

  triggers = {
    public_subnet   = "${cidrsubnet(var.vpc_cidr, var.newbits, var.az_number[data.aws_availability_zone.az.*.name_suffix[count.index]] + var.public_netnum_offset)}"
    private_subnets = "${cidrsubnet(var.vpc_cidr, var.newbits, var.az_number[data.aws_availability_zone.az.*.name_suffix[count.index]] + var.private_netnum_offset)}"
  }
}

output public_cidr_block {
  value = "${null_resource.subnets.*.triggers.public_subnet}"
}

output private_cidr_block {
  value = "${null_resource.subnets.*.triggers.private_subnets}"
}