Creating subnets in AWS using terraform with 'count', 'join' and 'element'

The following code snippets show how to create three private subnets in AWS using terraform with ‘count’, ‘join’ and ’element’:

To start with the following variables were declared and set:

terraform.tfvars

base_cidr_block = "192.168.0.0/16"
node_count      = 3
availability_zones = ["a", "b", "c"]

variables.tf

variable "base_cidr_block" {
  description = "The base of the address range to be used by the VPC and corresponding Subnets"
}

variable "availability_zones" {
  description = "A list containing 3 AZs"
  type        = list(string)
}

variable "node_count" {
  description = "The number of ec2 instances to deploy"
  type        = number
}

Three subnets are then created using a single resource block. The cidr_block parameter uses the cidr_block function, which (when combined with the count.index) creates three separate address ranges:

network.tf

resource "aws_subnet" "privlb_private_subnet" {
  count = 3 //Three subnets are required (across 3 AZs)  

  vpc_id = aws_vpc.ed_privlb_vpc.id
  cidr_block              = cidrsubnet(var.base_cidr_block, 8, count.index + 1)
  availability_zone       = join("", ["${var.region}", "${var.availability_zones[count.index]}"])
  map_public_ip_on_launch = false
}

To ensure that instances are then deployed evenly across subnets and availabilty zones, the element function is used:

instances.tf

resource "aws_instance" "ec2_instance" {
  count = var.node_count
  ami   = ami-123456789

  instance_type = t3.micro

  subnet_id = element(aws_subnet.privlb_private_subnet.*.id, count.index % 3)
}