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.
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module "subnets" {
source = "git::https://github.com/cloudposse/terraform-aws-dynamic-subnets.git?ref=master"
namespace = "cp"
stage = "prod"
name = "app"
region = "us-east-1"
vpc_id = "vpc-XXXXXXXX"
igw_id = "igw-XXXXXXXX"
cidr_block = "10.0.0.0/16"
availability_zones = ["us-east-1a", "us-east-1b"]
}
terraform-aws-dynamic-subnets
creates a set of subnets based on ${var.cidr_block}
input and number of Availability Zones in the region.
For subnet set calculation, the module uses Terraform interpolation
${
cidrsubnet(
signum(length(var.cidr_block)) == 1 ?
var.cidr_block : data.aws_vpc.default.cidr_block,
ceil(log(length(data.aws_availability_zones.available.names) * 2, 2)),
count.index)
}
-
Use
${var.cidr_block}
input (if specified) or use a VPC CIDR blockdata.aws_vpc.default.cidr_block
(e.g.10.0.0.0/16
) -
Get number of available AZ in the region (e.g.
length(data.aws_availability_zones.available.names)
) -
Calculate
newbits
.newbits
number specifies how many subnets be the CIDR block (input or VPC) will be divided into.newbits
is the number ofbinary digits
.Example:
newbits = 1
- 2 subnets are available (1 binary digit
allows to count up to2
)newbits = 2
- 4 subnets are available (2 binary digits
allows to count up to4
)newbits = 3
- 8 subnets are available (3 binary digits
allows to count up to8
)etc.
-
We know, that we have
6
AZs in aus-east-1
region (see step 2). -
We need to create
1 public
subnet and1 private
subnet in each AZ, thus we need to create12 subnets
in total (6
AZs * (1 public
+1 private
)). -
We need
4 binary digits
for that ( 24 = 16 ). In order to calculate the number ofbinary digits
we should uselogarithm
function. We should usebase 2
logarithm because decimal numbers can be calculated aspowers
of binary number. See Wiki for more detailsExample:
For
12 subnets
we need3.58
binary digits
(log212)For
16 subnets
we need4
binary digits
(log216)For
7 subnets
we need2.81
binary digits
(log27)etc.
-
We can't use fractional values to calculate the number of
binary digits
. We can't round it down because smaller number ofbinary digits
is insufficient to represent the required subnets. We round it up. See ceil.Example:
For
12 subnets
we need4
binary digits
(ceil(log212))For
16 subnets
we need4
binary digits
(ceil(log216))For
7 subnets
we need3
binary digits
(ceil(log27))etc.
-
Assign private subnets according to AZ number (we're using
count.index
for that). -
Assign public subnets according to AZ number but with a shift according to the number of AZs in the region (see step 2)
-
Available targets:
help Help screen
help/all Display help for all targets
help/short This help short screen
lint Lint terraform code
Name | Description | Type | Default | Required |
---|---|---|---|---|
attributes | Additional attributes (e.g. policy or role ) |
list | <list> |
no |
availability_zones | List of Availability Zones where subnets will be created | list | - | yes |
cidr_block | Base CIDR block which will be divided into subnet CIDR blocks (e.g. 10.0.0.0/16 ) |
string | - | yes |
delimiter | Delimiter to be used between namespace , stage , name , and attributes |
string | - |
no |
igw_id | Internet Gateway ID the public route table will point to (e.g. igw-9c26a123 ) |
string | - | yes |
map_public_ip_on_launch | Instances launched into a public subnet should be assigned a public IP address | string | true |
no |
max_subnet_count | Sets the maximum amount of subnets to deploy. 0 will deploy a subnet for every availablility zone within the region | string | 0 |
no |
name | Name (e.g. app ) |
string | - | yes |
namespace | Namespace (e.g. cp or cloudposse ) |
string | - | yes |
nat_gateway_enabled | Flag to enable/disable NAT gateways for private subnets | string | true |
no |
private_network_acl_id | Network ACL ID that will be added to private subnets. If empty, a new ACL will be created | string | `` | no |
public_network_acl_id | Network ACL ID that will be added to public subnets. If empty, a new ACL will be created | string | `` | no |
region | AWS Region (e.g. us-east-1 ) |
string | - | yes |
stage | Stage (e.g. prod , dev , staging ) |
string | - | yes |
subnet_type_tag_key | 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 | cpco.io/subnet/type |
no |
tags | Additional tags (e.g. map(Cluster ,XYZ ) |
map | <map> |
no |
vpc_default_route_table_id | Default route table for public subnets. If not set, will be created. (e.g. rtb-f4f0ce12 ) |
string | `` | no |
vpc_id | VPC ID where subnets will be created (e.g. vpc-aceb2723 ) |
string | - | yes |
Name | Description |
---|---|
availability_zones | List of Availability Zones where subnets were created |
nat_gateway_ids | AWS IDs of the NAT gateways created |
private_route_table_ids | AWS IDs of the created private route tables |
private_subnet_cidrs | CIDR blocks of the created private subnets |
private_subnet_ids | AWS IDs of the created private subnets |
public_route_table_ids | AWS IDs of the created public route tables |
public_subnet_cidrs | CIDR blocks of the created public subnets |
public_subnet_ids | AWS IDs of the created public subnets |
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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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Copyright © 2017-2019 Cloud Posse, LLC
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