GitHub - cloudposse/terraform-aws-dynamic-subnets: Terraform module for public and private subnets provisioning in existing VPC

Terraform module to provision public and private subnets in an existing VPC

Note: This module is intended for use with an existing VPC and existing Internet Gateway. To create a new VPC, use terraform-aws-vpc module.

Note: Due to Terraform limitations, many optional inputs to this module are specified as a list(string) that can have zero or one element, rather than as a string that could be empty or null. The designation of an input as a list type does not necessarily mean that you can supply more than one value in the list, so check the input's description before supplying more than one value.

The core function of this module is to create 2 sets of subnets, a "public" set with bidirectional access to the public internet, and a "private" set behind a firewall with egress-only access to the public internet. This includes dividing up a given CIDR range so that a each subnet gets its own distinct CIDR range within that range, and then creating those subnets in the appropriate availability zones. The intention is to keep this module relatively simple and easy to use for the most popular use cases. In its default configuration, this module creates 1 public subnet and 1 private subnet in each of the specified availability zones. The public subnets are configured for bi-directional traffic to the public internet, while the private subnets are configured for egress-only traffic to the public internet.

The module supports creating different numbers of public and private subnets per availability zone. This is useful for common architectures where you need a single public subnet for load balancers but multiple private subnets for different application tiers (web, app, data). You can specify the number and names of public and private subnets independently using public_subnets_per_az_count/public_subnets_per_az_names and private_subnets_per_az_count/private_subnets_per_az_names variables.

Rather than provide a wealth of configuration options allowing for numerous special cases, this module provides some common options and further provides the ability to suppress the creation of resources, allowing you to create and configure them as you like from outside this module. For example, rather than give you the option to customize the Network ACL, the module gives you the option to create a completely open one (and control access via Security Groups and other means) or not create one at all, allowing you to create and configure one yourself.

Public subnets

This module defines a public subnet as one that has direct access to an internet gateway and can accept incoming connection requests. In the simplest configuration, the module creates a single route table with a default route targeted to the VPC's internet gateway, and associates all the public subnets with that single route table.

Likewise it creates a single Network ACL with associated rules allowing all ingress and all egress, and associates that ACL with all the public subnets.

Private subnets

A private subnet may be able to initiate traffic to the public internet through a NAT gateway, a NAT instance, or an egress-only internet gateway, or it might only have direct access to other private subnets. In the simple configuration, for IPv4 and/or IPv6 with NAT64 enabled via public_dns64_enabled or private_dns64_enabled, the module creates 1 NAT Gateway or NAT Instance for each private subnet (in the public subnet in the same availability zone), creates 1 route table for each private subnet, and adds to that route table a default route from the subnet to its NAT Gateway or Instance. For IPv6, the module adds a route to the Egress-Only Internet Gateway configured via input.

As with the Public subnets, the module creates a single Network ACL with associated rules allowing all ingress and all egress, and associates that ACL with all the private subnets.

Customization for special use cases

Various features are controlled by bool inputs with names ending in _enabled. By changing the default values, you can enable or disable creation of public subnets, private subnets, route tables, NAT gateways, NAT instances, or Network ACLs. So for example, you could use this module to create only private subnets and the open Network ACL, and then add your own route table associations to the subnets and route all non-local traffic to a Transit Gateway or VPN.

CIDR allocation

For IPv4, you provide a CIDR and the module divides the address space into the largest CIDRs possible that are still small enough to accommodate max_subnet_count subnets of each enabled type (public or private). When max_subnet_count is left at the default 0, it is set to the total number of availability zones in the region. Private subnets are allocated out of the first half of the reserved range, and public subnets are allocated out of the second half.

For IPv6, you provide a /56 CIDR and the module assigns /64 subnets of that CIDR in consecutive order starting at zero. (You have the option of specifying a list of CIDRs instead.) As with IPv4, enough CIDRs are allocated to cover max_subnet_count private and public subnets (when both are enabled, which is the default), with the private subnets being allocated out of the lower half of the reservation and the public subnets allocated out of the upper half.

Deployment Modes and Configuration

This module supports various deployment modes through flexible configuration variables. Understanding these options allows you to tailor the subnet architecture to your specific use case.

Availability Zone Selection

availability_zones - Explicitly specify which AZs to use:

Provide a list of AZ names (e.g., ["us-east-1a", "us-east-1b", "us-east-1c"])
The list order must be stable - do not reorder or Terraform will recreate subnets
If empty, the module uses all available AZs in the region (sorted alphabetically)
Can be truncated by max_subnet_count if you specify more AZs than the limit

availability_zone_ids - Use AZ IDs instead of names:

Provide a list of AZ IDs (e.g., ["use1-az1", "use1-az2"])
Overrides availability_zones when set
Useful for multi-account consistency (AZ names like "us-east-1a" map to different physical locations across accounts, but AZ IDs are consistent)
The module automatically translates IDs to names for resource creation

Subnet Count and CIDR Reservation

max_subnet_count - Controls CIDR reservation for future growth:

Default: 0 (reserves CIDRs for all AZs in the region)
Recommended: Set to 3 or the maximum number of AZs you anticipate using
The module reserves CIDR space for this many subnets of each type (public and private)
Example: If a region has 4 AZs but you set max_subnet_count = 3, only 3 subnets will be created, but you can later expand to the 4th without changing existing subnet CIDRs
Important: This must be a constant value, not computed, due to Terraform limitations

subnets_per_az_count - Create multiple subnets of each type per AZ:

Default: 1 (one public and one private subnet per AZ)
Set to 2 or higher to create multiple subnets per AZ
Creates the same number of public and private subnets
Useful for segmenting workloads within the same AZ (e.g., separate subnets for web tier, app tier, data tier)
Each subnet gets its own CIDR from the allocated range
Works with subnets_per_az_names for organized outputs

subnets_per_az_names - Assign names to subnets for better organization:

Default: ["common"]
Provide a list of names matching subnets_per_az_count (e.g., ["web", "app", "data"])
Names are used as keys in the named_private_subnets_map and named_public_subnets_map outputs
Makes it easy to reference specific subnet groups in other modules
Example: module.subnets.named_private_subnets_map["web"] returns all web-tier private subnet IDs

public_subnets_per_az_count - Set a different number of public subnets per AZ:

Default: null (uses subnets_per_az_count for backward compatibility)
Set this when you need a different number of public vs private subnets
Common pattern: Set to 1 for a single public subnet (for load balancers) while having multiple private subnets
Must be greater than 0 if specified
Works independently from private_subnets_per_az_count

public_subnets_per_az_names - Assign names specifically to public subnets:

Default: null (uses subnets_per_az_names for backward compatibility)
Provide a list of names matching public_subnets_per_az_count
Names are used as keys in the named_public_subnets_map output
Example: ["public-lb"] for a single load balancer subnet per AZ

private_subnets_per_az_count - Set a different number of private subnets per AZ:

Default: null (uses subnets_per_az_count for backward compatibility)
Set this when you need a different number of private vs public subnets
Common pattern: Set to 3 for multi-tier architecture (web, app, data) while having only 1 public subnet
Must be greater than 0 if specified
Works independently from public_subnets_per_az_count

private_subnets_per_az_names - Assign names specifically to private subnets:

Default: null (uses subnets_per_az_names for backward compatibility)
Provide a list of names matching private_subnets_per_az_count
Names are used as keys in the named_private_subnets_map output
Example: ["web", "app", "data"] for a three-tier architecture

Subnet Type Selection

public_subnets_enabled - Enable/disable public subnet creation:

Default: true
Set to false to create only private subnets
When disabled, NAT Gateways/Instances are also disabled (since they require public subnets)
Use case: Internal-only VPCs that route through Transit Gateway or VPN

private_subnets_enabled - Enable/disable private subnet creation:

Default: true
Set to false to create only public subnets
When disabled, NAT Gateways/Instances are also disabled (since private subnets don't need them)
Use case: DMZ or edge VPCs with only internet-facing resources

NAT Configuration and Cost Optimization

max_nats - Limit the number of NAT devices for cost savings:

Default: 999 (creates one NAT per AZ for high availability)
Set to 1 for cost savings (single NAT, reduced availability)
Set to 2 for balance between cost and availability (two NATs across AZs)
Cost impact: Each NAT Gateway costs ~$32/month plus data transfer fees
Availability impact: If the NAT fails (or its AZ fails), private subnets lose internet access
The module distributes NAT devices across the first N availability zones
Example: With 3 AZs and max_nats = 1, only the first AZ gets a NAT Gateway

nat_gateway_public_subnet_indices - Control which public subnet gets the NAT Gateway (by index):

Default: [0] (place NAT in the first public subnet of each AZ)
When you have multiple public subnets per AZ, this determines which one hosts the NAT Gateway
NAT Gateways are shared - one NAT per AZ serves all private subnets in that AZ
Important: Each subnet index must be less than public_subnets_per_az_count
Example: With public_subnets_per_az_count = 2 and nat_gateway_public_subnet_indices = [0], the NAT goes in the first public subnet
Advanced: Set to [0, 1] to create redundant NATs within each AZ (rarely needed, increases cost)
Cannot be used with nat_gateway_public_subnet_names (choose indices OR names, not both)

nat_gateway_public_subnet_names - Control which public subnet gets the NAT Gateway (by name):

Default: null (uses nat_gateway_public_subnet_indices instead)
More intuitive alternative to using indices - specify subnets by name
References the names from public_subnets_per_az_names
Example: ["loadbalancer"] places NAT in the "loadbalancer" subnet
Example: ["loadbalancer", "web"] creates 2 NATs per AZ in both named subnets (expensive)
Cannot be used with nat_gateway_public_subnet_indices (choose indices OR names, not both)
Recommended approach for clarity and maintainability

Common Deployment Patterns

Standard HA deployment (default):

# 1 public + 1 private subnet per AZ, with NAT Gateway per AZ
public_subnets_enabled  = true
private_subnets_enabled = true
max_subnet_count        = 3  # Reserve space for 3 AZs
max_nats                = 999 # One NAT per AZ

Cost-optimized deployment:

# Single NAT Gateway shared across all private subnets
max_nats = 1
# Everything else default

Private-only with Transit Gateway:

# Private subnets only, routing through TGW
public_subnets_enabled  = false
private_subnets_enabled = true
nat_gateway_enabled     = false
# Add custom routes to TGW externally

Public-only (DMZ) deployment:

# Public subnets only for internet-facing resources
public_subnets_enabled  = true
private_subnets_enabled = false

Multi-tier architecture per AZ (legacy approach - same number of public and private):

# 3 private AND 3 public subnets per AZ (web, app, data)
subnets_per_az_count = 3
subnets_per_az_names = ["web", "app", "data"]
# Access subnets via: named_private_subnets_map["web"]

Multi-tier with separate public/private counts (recommended):

# 1 public subnet per AZ for load balancers
# 3 private subnets per AZ for web, app, and data tiers
public_subnets_per_az_count  = 1
public_subnets_per_az_names  = ["public-lb"]
private_subnets_per_az_count = 3
private_subnets_per_az_names = ["web", "app", "data"]
# NAT Gateway automatically goes in the first (and only) public subnet
# Can omit nat_gateway config since there's only one public subnet
# Access subnets via:
#   named_public_subnets_map["public-lb"]
#   named_private_subnets_map["web"]
#   named_private_subnets_map["app"]
#   named_private_subnets_map["data"]

Multiple public subnets with controlled NAT placement:

# 2 public subnets per AZ: one for ALB, one for bastion hosts
# Place NAT Gateway in the ALB subnet
public_subnets_per_az_count = 2
public_subnets_per_az_names = ["alb", "bastion"]

# OPTION 1: Use subnet name (recommended - more readable)
nat_gateway_public_subnet_names = ["alb"]

# OPTION 2: Use subnet index (alternative)
# nat_gateway_public_subnet_indices = [0]  # 0 = first subnet = "alb"

# Result: 1 NAT per AZ in the "alb" subnet, shared by all private subnets

Multiple private + multiple public with one NAT in specific subnet:

# Real-world example: Database, app tiers + load balancer and web frontends
# 3 private subnets per AZ (database, app1, app2)
# 2 public subnets per AZ (loadbalancer, web)
# 1 NAT Gateway per AZ in the "loadbalancer" subnet

availability_zones = ["us-east-1a", "us-east-1b", "us-east-1c"]

private_subnets_per_az_count = 3
private_subnets_per_az_names = ["database", "app1", "app2"]

public_subnets_per_az_count  = 2
public_subnets_per_az_names  = ["loadbalancer", "web"]

# Place NAT Gateway in the "loadbalancer" subnet (by name)
nat_gateway_public_subnet_names = ["loadbalancer"]

# Result per AZ:
# - 3 private subnets: database, app1, app2
# - 2 public subnets: loadbalancer, web
# - 1 NAT Gateway in "loadbalancer" subnet
# - All 3 private subnets route to the same NAT
# Total: 3 NAT Gateways (one per AZ) = ~$96/month

Multiple private + multiple public with NAT in EACH public subnet (high availability):

# Advanced: Redundant NAT Gateways within each AZ for maximum availability
# 3 private subnets per AZ (database, app1, app2)
# 2 public subnets per AZ (loadbalancer, web)
# 2 NAT Gateways per AZ (one in each public subnet)

availability_zones = ["us-east-1a", "us-east-1b", "us-east-1c"]

private_subnets_per_az_count = 3
private_subnets_per_az_names = ["database", "app1", "app2"]

public_subnets_per_az_count  = 2
public_subnets_per_az_names  = ["loadbalancer", "web"]

# Place NAT Gateways in BOTH public subnets (by name)
nat_gateway_public_subnet_names = ["loadbalancer", "web"]

# Alternative using indices:
# nat_gateway_public_subnet_indices = [0, 1]

# Result per AZ:
# - 3 private subnets: database, app1, app2
# - 2 public subnets: loadbalancer, web
# - 2 NAT Gateways: one in "loadbalancer", one in "web"
# - Private subnets distributed across NATs:
#   - "database" → NAT in "loadbalancer"
#   - "app1" → NAT in "web"
#   - "app2" → NAT in "loadbalancer"
# Total: 6 NAT Gateways (2 per AZ × 3 AZs) = ~$192/month
# WARNING: This is expensive. Use only if you need intra-AZ NAT redundancy.

NAT Gateway ID References in Outputs

The module exposes NAT Gateway IDs in the subnet stats outputs, enabling downstream components like network firewalls to reference the NAT Gateways associated with each subnet.

named_private_subnets_stats_map - Each private subnet includes the NAT Gateway ID it routes to:

# Output structure (4 fields per subnet):
named_private_subnets_stats_map = {
  "database" = [
    {
      az             = "us-east-2a"
      subnet_id      = "subnet-abc123"
      route_table_id = "rtb-def456"
      nat_gateway_id = "nat-xyz789"  # NAT Gateway this subnet routes to for egress
    },
    # ... one entry per AZ
  ]
  "app1" = [ ... ]
  "app2" = [ ... ]
}

named_public_subnets_stats_map - Each public subnet includes the NAT Gateway ID if one exists in that subnet:

# Output structure (4 fields per subnet):
named_public_subnets_stats_map = {
  "loadbalancer" = [
    {
      az             = "us-east-2a"
      subnet_id      = "subnet-ghi789"
      route_table_id = "rtb-jkl012"
      nat_gateway_id = "nat-xyz789"  # NAT Gateway in this public subnet (if any)
    },
    # ... one entry per AZ
  ]
  "web" = [ ... ]
}

Use case example - Network firewall routing:

# Reference NAT Gateway IDs from subnet stats
locals {
  database_nat_gateways = [
    for stats in module.subnets.named_private_subnets_stats_map["database"] :
    stats.nat_gateway_id if stats.nat_gateway_id != ""
  ]
}

# Use in network firewall route configuration
resource "aws_networkfirewall_firewall_policy" "example" {
  # ... configuration that needs NAT Gateway IDs
}

Multi-account with consistent AZs:

# Use AZ IDs for consistency across accounts
availability_zone_ids = ["use1-az1", "use1-az2", "use1-az4"]
# These map to the same physical locations across all accounts

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Usage

module "subnets" {
  source = "cloudposse/dynamic-subnets/aws"
  # Cloud Posse recommends pinning every module to a specific version
  # version = "x.x.x"
  namespace           = "eg"
  stage               = "prod"
  name                = "app"
  vpc_id              = "vpc-XXXXXXXX"
  igw_id              = ["igw-XXXXXXXX"]
  ipv4_cidr_block     = ["10.0.0.0/16"]
  availability_zones  = ["us-east-1a", "us-east-1b"]
}

Create only private subnets, route to transit gateway:

module "private_tgw_subnets" {
  source = "cloudposse/dynamic-subnets/aws"
  # Cloud Posse recommends pinning every module to a specific version
  # version = "x.x.x"
  namespace           = "eg"
  stage               = "prod"
  name                = "app"
  vpc_id              = "vpc-XXXXXXXX"
  igw_id              = ["igw-XXXXXXXX"]
  ipv4_cidr_block     = ["10.0.0.0/16"]
  availability_zones  = ["us-east-1a", "us-east-1b"]

  nat_gateway_enabled    = false
  public_subnets_enabled = false
}

resource "aws_route" "private" {
  count = length(module.private_tgw_subnets.private_route_table_ids)

  route_table_id         = module.private_tgw_subnets.private_route_table_ids[count.index]
  destination_cidr_block = "0.0.0.0/0"
  transit_gateway_id     = "tgw-XXXXXXXXX"
}

See examples for working examples. In particular, see examples/nacls for an example of how to create custom Network Access Control Lists (NACLs) outside of but in conjunction with this module.

Important

In Cloud Posse's examples, we avoid pinning modules to specific versions to prevent discrepancies between the documentation and the latest released versions. However, for your own projects, we strongly advise pinning each module to the exact version you're using. This practice ensures the stability of your infrastructure. Additionally, we recommend implementing a systematic approach for updating versions to avoid unexpected changes.

Requirements

Name	Version
terraform	>= 1.1.0
aws	>= 5.0

Providers

Name	Version
aws	>= 5.0

Modules

Name	Source	Version
nat_instance_label	cloudposse/label/null	0.25.0
nat_label	cloudposse/label/null	0.25.0
private_label	cloudposse/label/null	0.25.0
public_label	cloudposse/label/null	0.25.0
this	cloudposse/label/null	0.25.0
utils	cloudposse/utils/aws	1.4.0

Resources

Name	Type
aws_eip.default	resource
aws_eip_association.nat_instance	resource
aws_instance.nat_instance	resource
aws_nat_gateway.default	resource
aws_network_acl.private	resource
aws_network_acl.public	resource
aws_network_acl_rule.private4_egress	resource
aws_network_acl_rule.private4_ingress	resource
aws_network_acl_rule.private6_egress	resource
aws_network_acl_rule.private6_ingress	resource
aws_network_acl_rule.public4_egress	resource
aws_network_acl_rule.public4_ingress	resource
aws_network_acl_rule.public6_egress	resource
aws_network_acl_rule.public6_ingress	resource
aws_route.nat4	resource
aws_route.nat_instance	resource
aws_route.private6	resource
aws_route.private_nat64	resource
aws_route.public	resource
aws_route.public6	resource
aws_route.public_nat64	resource
aws_route_table.private	resource
aws_route_table.public	resource
aws_route_table_association.private	resource
aws_route_table_association.public	resource
aws_security_group.nat_instance	resource
aws_security_group_rule.nat_instance_egress	resource
aws_security_group_rule.nat_instance_ingress	resource
aws_subnet.private	resource
aws_subnet.public	resource
aws_ami.nat_instance	data source
aws_availability_zones.default	data source
aws_eip.nat	data source
aws_vpc.default	data source

Inputs

Name	Description	Type	Default	Required
additional_tag_map	Additional key-value pairs to add to each map in `tags_as_list_of_maps`. Not added to `tags` or `id`. This is for some rare cases where resources want additional configuration of tags and therefore take a list of maps with tag key, value, and additional configuration.	`map(string)`	`{}`	no
attributes	ID element. Additional attributes (e.g. `workers` or `cluster`) to add to `id`, in the order they appear in the list. New attributes are appended to the end of the list. The elements of the list are joined by the `delimiter` and treated as a single ID element.	`list(string)`	`[]`	no
availability_zone_attribute_style	The style of Availability Zone code to use in tags and names. One of `full`, `short`, or `fixed`. When using `availability_zone_ids`, IDs will first be translated into AZ names.	`string`	`"short"`	no
availability_zone_ids	List of Availability Zones IDs where subnets will be created. Overrides `availability_zones`. Useful in some regions when using only some AZs and you want to use the same ones across multiple accounts.	`list(string)`	`[]`	no
availability_zones	List of Availability Zones (AZs) where subnets will be created. Ignored when `availability_zone_ids` is set. The order of zones in the list must be stable or else Terraform will continually make changes. If no AZs are specified, then `max_subnet_count` AZs will be selected in alphabetical order. If `max_subnet_count > 0` and `length(var.availability_zones) > max_subnet_count`, the list will be truncated. We recommend setting `availability_zones` and `max_subnet_count` explicitly as constant (not computed) values for predictability, consistency, and stability.	`list(string)`	`[]`	no
aws_route_create_timeout	DEPRECATED: Use `route_create_timeout` instead. Time to wait for AWS route creation, specified as a Go Duration, e.g. `2m`	`string`	`null`	no
aws_route_delete_timeout	DEPRECATED: Use `route_delete_timeout` instead. Time to wait for AWS route deletion, specified as a Go Duration, e.g. `2m`	`string`	`null`	no
context	Single object for setting entire context at once. See description of individual variables for details. Leave string and numeric variables as `null` to use default value. Individual variable settings (non-null) override settings in context object, except for attributes, tags, and additional_tag_map, which are merged.	`any`	{ "additional_tag_map": {}, "attributes": [], "delimiter": null, "descriptor_formats": {}, "enabled": true, "environment": null, "id_length_limit": null, "label_key_case": null, "label_order": [], "label_value_case": null, "labels_as_tags": [ "unset" ], "name": null, "namespace": null, "regex_replace_chars": null, "stage": null, "tags": {}, "tenant": null }	no
delimiter	Delimiter to be used between ID elements. Defaults to `-` (hyphen). Set to `""` to use no delimiter at all.	`string`	`null`	no
descriptor_formats	Describe additional descriptors to be output in the `descriptors` output map. Map of maps. Keys are names of descriptors. Values are maps of the form `{<br/> format = string<br/> labels = list(string)<br/>}` (Type is `any` so the map values can later be enhanced to provide additional options.) `format` is a Terraform format string to be passed to the `format()` function. `labels` is a list of labels, in order, to pass to `format()` function. Label values will be normalized before being passed to `format()` so they will be identical to how they appear in `id`. Default is `{}` (`descriptors` output will be empty).	`any`	`{}`	no
enabled	Set to false to prevent the module from creating any resources	`bool`	`null`	no
environment	ID element. Usually used for region e.g. 'uw2', 'us-west-2', OR role 'prod', 'staging', 'dev', 'UAT'	`string`	`null`	no
id_length_limit	Limit `id` to this many characters (minimum 6). Set to `0` for unlimited length. Set to `null` for keep the existing setting, which defaults to `0`. Does not affect `id_full`.	`number`	`null`	no
igw_id	The Internet Gateway ID that the public subnets will route traffic to. Used if `public_route_table_enabled` is `true`, ignored otherwise.	`list(string)`	`[]`	no
ipv4_cidr_block	Base IPv4 CIDR block which will be divided into subnet CIDR blocks (e.g. `10.0.0.0/16`). Ignored if `ipv4_cidrs` is set. If no CIDR block is provided, the VPC's default IPv4 CIDR block will be used.	`list(string)`	`[]`	no
ipv4_cidrs	Lists of CIDRs to assign to subnets. Order of CIDRs in the lists must not change over time. Lists may contain more CIDRs than needed.	list(object({ private = list(string) public = list(string) }))	`[]`	no
ipv4_enabled	Set `true` to enable IPv4 addresses in the subnets	`bool`	`true`	no
ipv4_private_instance_hostname_type	How to generate the DNS name for the instances in the private subnets. Either `ip-name` to generate it from the IPv4 address, or `resource-name` to generate it from the instance ID.	`string`	`"ip-name"`	no
ipv4_private_instance_hostnames_enabled	If `true`, DNS queries for instance hostnames in the private subnets will be answered with A (IPv4) records.	`bool`	`false`	no
ipv4_public_instance_hostname_type	How to generate the DNS name for the instances in the public subnets. Either `ip-name` to generate it from the IPv4 address, or `resource-name` to generate it from the instance ID.	`string`	`"ip-name"`	no
ipv4_public_instance_hostnames_enabled	If `true`, DNS queries for instance hostnames in the public subnets will be answered with A (IPv4) records.	`bool`	`false`	no
ipv6_cidr_block	Base IPv6 CIDR block from which `/64` subnet CIDRs will be assigned. Must be `/56`. (e.g. `2600:1f16:c52:ab00::/56`). Ignored if `ipv6_cidrs` is set. If no CIDR block is provided, the VPC's default IPv6 CIDR block will be used.	`list(string)`	`[]`	no
ipv6_cidrs	Lists of CIDRs to assign to subnets. Order of CIDRs in the lists must not change over time. Lists may contain more CIDRs than needed.	list(object({ private = list(string) public = list(string) }))	`[]`	no
ipv6_egress_only_igw_id	The Egress Only Internet Gateway ID the private IPv6 subnets will route traffic to. Used if `private_route_table_enabled` is `true` and `ipv6_enabled` is `true`, ignored otherwise.	`list(string)`	`[]`	no
ipv6_enabled	Set `true` to enable IPv6 addresses in the subnets	`bool`	`false`	no
ipv6_private_instance_hostnames_enabled	If `true` (or if `ipv4_enabled` is `false`), DNS queries for instance hostnames in the private subnets will be answered with AAAA (IPv6) records.	`bool`	`false`	no
ipv6_public_instance_hostnames_enabled	If `true` (or if `ipv4_enabled` is false), DNS queries for instance hostnames in the public subnets will be answered with AAAA (IPv6) records.	`bool`	`false`	no
label_key_case	Controls the letter case of the `tags` keys (label names) for tags generated by this module. Does not affect keys of tags passed in via the `tags` input. Possible values: `lower`, `title`, `upper`. Default value: `title`.	`string`	`null`	no
label_order	The order in which the labels (ID elements) appear in the `id`. Defaults to ["namespace", "environment", "stage", "name", "attributes"]. You can omit any of the 6 labels ("tenant" is the 6th), but at least one must be present.	`list(string)`	`null`	no
label_value_case	Controls the letter case of ID elements (labels) as included in `id`, set as tag values, and output by this module individually. Does not affect values of tags passed in via the `tags` input. Possible values: `lower`, `title`, `upper` and `none` (no transformation). Set this to `title` and set `delimiter` to `""` to yield Pascal Case IDs. Default value: `lower`.	`string`	`null`	no
labels_as_tags	Set of labels (ID elements) to include as tags in the `tags` output. Default is to include all labels. Tags with empty values will not be included in the `tags` output. Set to `[]` to suppress all generated tags. Notes: The value of the `name` tag, if included, will be the `id`, not the `name`. Unlike other `null-label` inputs, the initial setting of `labels_as_tags` cannot be changed in later chained modules. Attempts to change it will be silently ignored.	`set(string)`	[ "default" ]	no
map_public_ip_on_launch	If `true`, instances launched into a public subnet will be assigned a public IPv4 address	`bool`	`true`	no
max_nats	Upper limit on number of NAT Gateways/Instances to create. Set to 1 or 2 for cost savings at the expense of availability.	`number`	`999`	no
max_subnet_count	Sets the maximum number of each type (public or private) of subnet to deploy. `0` will reserve a CIDR for every Availability Zone (excluding Local Zones) in the region, and deploy a subnet in each availability zone specified in `availability_zones` or `availability_zone_ids`, or every zone if none are specified. We recommend setting this equal to the maximum number of AZs you anticipate using, to avoid causing subnets to be destroyed and recreated with smaller IPv4 CIDRs when AWS adds an availability zone. Due to Terraform limitations, you can not set `max_subnet_count` from a computed value, you have to set it from an explicit constant. For most cases, `3` is a good choice.	`number`	`0`	no
metadata_http_endpoint_enabled	Whether the metadata service is available on the created NAT instances	`bool`	`true`	no
metadata_http_put_response_hop_limit	The desired HTTP PUT response hop limit (between 1 and 64) for instance metadata requests on the created NAT instances	`number`	`1`	no
metadata_http_tokens_required	Whether or not the metadata service requires session tokens, also referred to as Instance Metadata Service Version 2, on the created NAT instances	`bool`	`true`	no
name	ID element. Usually the component or solution name, e.g. 'app' or 'jenkins'. This is the only ID element not also included as a `tag`. The "name" tag is set to the full `id` string. There is no tag with the value of the `name` input.	`string`	`null`	no
namespace	ID element. Usually an abbreviation of your organization name, e.g. 'eg' or 'cp', to help ensure generated IDs are globally unique	`string`	`null`	no
nat_elastic_ips	Existing Elastic IPs (not EIP IDs) to attach to the NAT Gateway(s) or Instance(s) instead of creating new ones.	`list(string)`	`[]`	no
nat_gateway_enabled	Set `true` to create NAT Gateways to perform IPv4 NAT and NAT64 as needed. Defaults to `true` unless `nat_instance_enabled` is `true`.	`bool`	`null`	no
nat_gateway_public_subnet_indices	The index (starting from 0) of the public subnet in each AZ to place the NAT Gateway. If you have multiple public subnets per AZ (via `public_subnets_per_az_count`), this determines which one gets the NAT Gateway. Default: `[0]` (use the first public subnet in each AZ). You can specify multiple indices if you want redundant NATs within an AZ, but this is rarely needed and increases cost. Cannot be used together with `nat_gateway_public_subnet_names`. Example: `[0]` creates 1 NAT per AZ in the first public subnet. Example: `[0, 1]` creates 2 NATs per AZ in the first and second public subnets (expensive).	`list(number)`	[ 0 ]	no
nat_gateway_public_subnet_names	The names of the public subnets in each AZ where NAT Gateways should be placed. Uses the names from `public_subnets_per_az_names` to determine placement. This is more intuitive than using indices - specify the subnet by name instead of position. Cannot be used together with `nat_gateway_public_subnet_indices` (only use indices OR names, not both). If not specified, defaults to using `nat_gateway_public_subnet_indices`. Example: `["loadbalancer"]` creates 1 NAT per AZ in the "loadbalancer" subnet. Example: `["loadbalancer", "web"]` creates 2 NATs per AZ in "loadbalancer" and "web" subnets (expensive).	`list(string)`	`null`	no
nat_instance_ami_id	A list optionally containing the ID of the AMI to use for the NAT instance. If the list is empty (the default), the latest official AWS NAT instance AMI will be used. NOTE: The Official NAT instance AMI is being phased out and does not support NAT64. Use of a NAT gateway is recommended instead.	`list(string)`	`[]`	no
nat_instance_cpu_credits_override	NAT Instance credit option for CPU usage. Valid values are "standard" or "unlimited". T3 and later instances are launched as unlimited by default. T2 instances are launched as standard by default.	`string`	`""`	no
nat_instance_enabled	Set `true` to create NAT Instances to perform IPv4 NAT. Defaults to `false`.	`bool`	`null`	no
nat_instance_root_block_device_encrypted	Whether to encrypt the root block device on the created NAT instances	`bool`	`true`	no
nat_instance_type	NAT Instance type	`string`	`"t3.micro"`	no
open_network_acl_ipv4_rule_number	The `rule_no` assigned to the network ACL rules for IPv4 traffic generated by this module	`number`	`100`	no
open_network_acl_ipv6_rule_number	The `rule_no` assigned to the network ACL rules for IPv6 traffic generated by this module	`number`	`111`	no
private_assign_ipv6_address_on_creation	If `true`, network interfaces created in a private subnet will be assigned an IPv6 address	`bool`	`true`	no
private_dns64_nat64_enabled	If `true` and IPv6 is enabled, DNS queries made to the Amazon-provided DNS Resolver in private subnets will return synthetic IPv6 addresses for IPv4-only destinations, and these addresses will be routed to the NAT Gateway. Requires `public_subnets_enabled`, `nat_gateway_enabled`, and `private_route_table_enabled` to be `true` to be fully operational. Defaults to `true` unless there is no public IPv4 subnet for egress, in which case it defaults to `false`.	`bool`	`null`	no
private_label	The string to use in IDs and elsewhere to identify resources for the private subnets and distinguish them from resources for the public subnets	`string`	`"private"`	no
private_open_network_acl_enabled	If `true`, a single network ACL be created and it will be associated with every private subnet, and a rule (number 100) will be created allowing all ingress and all egress. You can add additional rules to this network ACL using the `aws_network_acl_rule` resource. If `false`, you will need to manage the network ACL outside of this module.	`bool`	`true`	no
private_route_table_enabled	If `true`, a network route table and default route to the NAT gateway, NAT instance, or egress-only gateway will be created for each private subnet (1:1). If false, you will need to create your own route table(s) and route(s).	`bool`	`true`	no
private_subnets_additional_tags	Additional tags to be added to private subnets	`map(string)`	`{}`	no
private_subnets_enabled	If false, do not create private subnets (or NAT gateways or instances)	`bool`	`true`	no
private_subnets_per_az_count	The number of private subnets to provision per Availability Zone. If not provided, defaults to the value of `subnets_per_az_count` for backward compatibility. Set this to create a different number of private subnets than public subnets.	`number`	`null`	no
private_subnets_per_az_names	The names to assign to the private subnets per Availability Zone. If not provided, defaults to the value of `subnets_per_az_names` for backward compatibility. If provided, the length must match `private_subnets_per_az_count`. The names will be used as keys in the outputs `named_private_subnets_map` and `named_private_route_table_ids_map`.	`list(string)`	`null`	no
public_assign_ipv6_address_on_creation	If `true`, network interfaces created in a public subnet will be assigned an IPv6 address	`bool`	`true`	no
public_dns64_nat64_enabled	If `true` and IPv6 is enabled, DNS queries made to the Amazon-provided DNS Resolver in public subnets will return synthetic IPv6 addresses for IPv4-only destinations, and these addresses will be routed to the NAT Gateway. Requires `nat_gateway_enabled` and `public_route_table_enabled` to be `true` to be fully operational.	`bool`	`false`	no
public_label	The string to use in IDs and elsewhere to identify resources for the public subnets and distinguish them from resources for the private subnets	`string`	`"public"`	no
public_open_network_acl_enabled	If `true`, a single network ACL be created and it will be associated with every public subnet, and a rule will be created allowing all ingress and all egress. You can add additional rules to this network ACL using the `aws_network_acl_rule` resource. If `false`, you will need to manage the network ACL outside of this module.	`bool`	`true`	no
public_route_table_enabled	If `true`, network route table(s) will be created as determined by `public_route_table_per_subnet_enabled` and appropriate routes will be added to destinations this module knows about. If `false`, you will need to create your own route table(s) and route(s). Ignored if `public_route_table_ids` is non-empty.	`bool`	`true`	no
public_route_table_ids	List optionally containing the ID of a single route table shared by all public subnets or exactly one route table ID for each public subnet. If provided, it overrides `public_route_table_per_subnet_enabled`. If omitted and `public_route_table_enabled` is `true`, one or more network route tables will be created for the public subnets, according to the setting of `public_route_table_per_subnet_enabled`.	`list(string)`	`[]`	no
public_route_table_per_subnet_enabled	If `true` (and `public_route_table_enabled` is `true`), a separate network route table will be created for and associated with each public subnet. If `false` (and `public_route_table_enabled` is `true`), a single network route table will be created and it will be associated with every public subnet. If not set, it will be set to the value of `public_dns64_nat64_enabled`.	`bool`	`null`	no
public_subnets_additional_tags	Additional tags to be added to public subnets	`map(string)`	`{}`	no
public_subnets_enabled	If false, do not create public subnets. Since NAT gateways and instances must be created in public subnets, these will also not be created when `false`.	`bool`	`true`	no
public_subnets_per_az_count	The number of public subnets to provision per Availability Zone. If not provided, defaults to the value of `subnets_per_az_count` for backward compatibility. Set this to create a different number of public subnets than private subnets.	`number`	`null`	no
public_subnets_per_az_names	The names to assign to the public subnets per Availability Zone. If not provided, defaults to the value of `subnets_per_az_names` for backward compatibility. If provided, the length must match `public_subnets_per_az_count`. The names will be used as keys in the outputs `named_public_subnets_map` and `named_public_route_table_ids_map`.	`list(string)`	`null`	no
regex_replace_chars	Terraform regular expression (regex) string. Characters matching the regex will be removed from the ID elements. If not set, `"/[^a-zA-Z0-9-]/"` is used to remove all characters other than hyphens, letters and digits.	`string`	`null`	no
root_block_device_encrypted	DEPRECATED: use `nat_instance_root_block_device_encrypted` instead. Whether to encrypt the root block device on the created NAT instances	`bool`	`null`	no
route_create_timeout	Time to wait for a network routing table entry to be created, specified as a Go Duration, e.g. `2m`. Use `null` for proivder default.	`string`	`null`	no
route_delete_timeout	Time to wait for a network routing table entry to be deleted, specified as a Go Duration, e.g. `2m`. Use `null` for proivder default.	`string`	`null`	no
stage	ID element. Usually used to indicate role, e.g. 'prod', 'staging', 'source', 'build', 'test', 'deploy', 'release'	`string`	`null`	no
subnet_create_timeout	Time to wait for a subnet to be created, specified as a Go Duration, e.g. `2m`. Use `null` for proivder default.	`string`	`null`	no
subnet_delete_timeout	Time to wait for a subnet to be deleted, specified as a Go Duration, e.g. `5m`. Use `null` for proivder default.	`string`	`null`	no
subnet_type_tag_key	DEPRECATED: Use `public_subnets_additional_tags` and `private_subnets_additional_tags` instead Key for subnet type tag to provide information about the type of subnets, e.g. `cpco.io/subnet/type: private` or `cpco.io/subnet/type: public`	`string`	`null`	no
subnet_type_tag_value_format	DEPRECATED: Use `public_subnets_additional_tags` and `private_subnets_additional_tags` instead. The value of the `subnet_type_tag_key` will be set to `format(var.subnet_type_tag_value_format, <type>)` where `<type>` is either `public` or `private`.	`string`	`"%s"`	no
subnets_per_az_count	The number of subnet of each type (public or private) to provision per Availability Zone.	`number`	`1`	no
subnets_per_az_names	The subnet names of each type (public or private) to provision per Availability Zone. This variable is optional. If a list of names is provided, the list items will be used as keys in the outputs `named_private_subnets_map`, `named_public_subnets_map`, `named_private_route_table_ids_map` and `named_public_route_table_ids_map`	`list(string)`	[ "common" ]	no
tags	Additional tags (e.g. `{'BusinessUnit': 'XYZ'}`). Neither the tag keys nor the tag values will be modified by this module.	`map(string)`	`{}`	no
tenant	ID element _(Rarely used, not included by default)_. A customer identifier, indicating who this instance of a resource is for	`string`	`null`	no
vpc_id	VPC ID where subnets will be created (e.g. `vpc-aceb2723`)	`string`	n/a	yes

Outputs

Name	Description
availability_zone_ids	List of Availability Zones IDs where subnets were created, when available
availability_zones	List of Availability Zones where subnets were created
az_private_route_table_ids_map	Map of AZ names to list of private route table IDs in the AZs
az_private_subnets_map	Map of AZ names to list of private subnet IDs in the AZs
az_public_route_table_ids_map	Map of AZ names to list of public route table IDs in the AZs
az_public_subnets_map	Map of AZ names to list of public subnet IDs in the AZs
named_private_route_table_ids_map	Map of subnet names (specified in `private_subnets_per_az_names` or `subnets_per_az_names` variable) to lists of private route table IDs
named_private_subnets_map	Map of subnet names (specified in `private_subnets_per_az_names` or `subnets_per_az_names` variable) to lists of private subnet IDs
named_private_subnets_stats_map	Map of subnet names (specified in `private_subnets_per_az_names` or `subnets_per_az_names` variable) to lists of objects with each object having four items: AZ, private subnet ID, private route table ID, NAT Gateway ID (the NAT Gateway that this private subnet routes to for egress)
named_public_route_table_ids_map	Map of subnet names (specified in `public_subnets_per_az_names` or `subnets_per_az_names` variable) to lists of public route table IDs
named_public_subnets_map	Map of subnet names (specified in `public_subnets_per_az_names` or `subnets_per_az_names` variable) to lists of public subnet IDs
named_public_subnets_stats_map	Map of subnet names (specified in `public_subnets_per_az_names` or `subnets_per_az_names` variable) to lists of objects with each object having four items: AZ, public subnet ID, public route table ID, NAT Gateway ID (the NAT Gateway in this public subnet, if any)
nat_eip_allocation_ids	Elastic IP allocations in use by NAT
nat_gateway_ids	IDs of the NAT Gateways created
nat_gateway_public_ips	DEPRECATED: use `nat_ips` instead. Public IPv4 IP addresses in use by NAT.
nat_instance_ami_id	ID of AMI used by NAT instance
nat_instance_ids	IDs of the NAT Instances created
nat_ips	Elastic IP Addresses in use by NAT
private_network_acl_id	ID of the Network ACL created for private subnets
private_route_table_ids	IDs of the created private route tables
private_subnet_arns	ARNs of the created private subnets
private_subnet_cidrs	IPv4 CIDR blocks of the created private subnets
private_subnet_ids	IDs of the created private subnets
private_subnet_ipv6_cidrs	IPv6 CIDR blocks of the created private subnets
public_network_acl_id	ID of the Network ACL created for public subnets
public_route_table_ids	IDs of the created public route tables
public_subnet_arns	ARNs of the created public subnets
public_subnet_cidrs	IPv4 CIDR blocks of the created public subnets
public_subnet_ids	IDs of the created public subnets
public_subnet_ipv6_cidrs	IPv6 CIDR blocks of the created public subnets

Subnet calculation logic

terraform-aws-dynamic-subnets creates a set of subnets based on various CIDR inputs and the maximum possible number of subnets, which is max_subnet_count when specified or the number of Availability Zones in the region when max_subnet_count is left at its default value of zero.

You can explicitly provide CIDRs for subnets via ipv4_cidrs and ipv6_cidrs inputs if you want, but the usual use case is to provide a single CIDR which this module will subdivide into a set of CIDRs as follows:

Get number of available AZ in the region:

existing_az_count = length(data.aws_availability_zones.available.names)

Determine how many sets of subnets are being created. (Usually it is 2: public and private): subnet_type_count.
Multiply the results of (1) and (2) to determine how many CIDRs to reserve:

cidr_count = existing_az_count * subnet_type_count

Calculate the number of bits needed to enumerate all the CIDRs:

subnet_bits = ceil(log(cidr_count, 2))

Reserve CIDRs for private subnets using cidrsubnet:

private_subnet_cidrs = [ for netnumber in range(0, existing_az_count): cidrsubnet(cidr_block, subnet_bits, netnumber) ]

Reserve CIDRs for public subnets in the second half of the CIDR block:

public_subnet_cidrs = [ for netnumber in range(existing_az_count, existing_az_count * 2): cidrsubnet(cidr_block, subnet_bits, netnumber) ]

Note that this means that, for example, in a region with 4 availability zones, if you specify only 3 availability zones in var.availability_zones, this module will still reserve CIDRs for the 4th zone. This is so that if you later want to expand into that zone, the existing subnet CIDR assignments will not be disturbed. If you do not want to reserve these CIDRs, set max_subnet_count to the number of zones you are actually using.

Related Projects

Check out these related projects.

terraform-aws-vpc - Terraform Module that defines a VPC with public/private subnets across multiple AZs with Internet Gateways
terraform-aws-vpc-peering - Terraform module to create a peering connection between two VPCs
terraform-aws-kops-vpc-peering - Terraform module to create a peering connection between a backing services VPC and a VPC created by Kops
terraform-aws-named-subnets - Terraform module for named subnets provisioning.

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examples		examples
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README.yaml		README.yaml
atmos.yaml		atmos.yaml
context.tf		context.tf
main.tf		main.tf
moved.tf		moved.tf
nat-gateway.tf		nat-gateway.tf
nat-instance.tf		nat-instance.tf
outputs-deprecated.tf		outputs-deprecated.tf
outputs.tf		outputs.tf
private.tf		private.tf
public.tf		public.tf
variables-deprecated.tf		variables-deprecated.tf
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Public subnets

Private subnets

Customization for special use cases

CIDR allocation

Deployment Modes and Configuration

Availability Zone Selection

Subnet Count and CIDR Reservation

Subnet Type Selection

NAT Configuration and Cost Optimization

Common Deployment Patterns

NAT Gateway ID References in Outputs

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