Terraform CIDR Subnets Module

This is a simple Terraform module for calculating subnet addresses under a particular CIDR prefix.

Based on Hashicorp Terraform CIDR Subnets Module but with the improvements like adding extra fields and producing complex outputs so it can be used with for_each to create resources

It also includes an output to files for better understanding what is changing and also can be used for documentation purposes (e.g. JSON to Markdown Table)

Inputs

Name	Description	Type	Default	Required
base_cidr_block	A network address prefix in CIDR notation that all of the requested subnetwork prefixes will be allocated within.	string	n/a	yes
json_maps	Write the subnet maps to JSON files	bool	false	no
networks	A list of objects describing requested subnetwork prefixes.	list(object({ availability_zone = string map_public_ip_on_launch = bool new_bits = number role = string service = string }))	[]	no
separator_char	Separator character to join elements in order to build a single key on maps.	string	":"	no

Outputs

Name	Description
network_cidr_blocks	Map of networks
networks	List of maps
networks_complex_map	Complex maps of the networks
networks_map	Networks maps
networks_service_role_list	Maps of lists per service/role pair
networks_service_role_map	Maps of maps per service/role pair

Usage

Below if can find the usage of this module with some example.

To "remove" (mark as free) a subnet from the list do not delete from it, if so it will cause an move of all subnets CIDR's. To simply mark as free to use, replace one of the values of the files service, role or availability_zone with null value.

Module instantiation

module "subnet_addrs" {
  source = "marceloalmeida/subnets/cidr"

  base_cidr_block = "10.0.0.0/16"

  networks = [
    {
      service                 = "base"
      role                    = "general"
      new_bits                = 10
      map_public_ip_on_launch = false
      availability_zone       = "us-east-1a"
    },
    {
      service                 = "base"
      role                    = "general"
      new_bits                = 10
      map_public_ip_on_launch = true
      availability_zone       = "us-east-1b"
    },
    {
      service                 = "base"
      role                    = "general"
      new_bits                = 10
      map_public_ip_on_launch = false
      availability_zone       = "us-east-1b"
    },
    {
      service                 = null
      role                    = "general"
      new_bits                = 10
      map_public_ip_on_launch = true
      availability_zone       = "us-east-1a"
    },
    {
      service                 = "rds"
      role                    = "general"
      new_bits                = 10
      map_public_ip_on_launch = true
      availability_zone       = "us-east-1b"
    },
  ]
}

AWS subnet creation

resource "aws_subnet" "base_general" {
  for_each = lookup(module.subnets.networks_service_role_map, "base:general")

  vpc_id     = var.vpc_id
  cidr_block = each.value.cidr_block

  tags = each.value
}