terraform-aws-dynamic-subnets

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.

This project is part of our comprehensive "SweetOps" approach towards DevOps.

It's 100% Open Source and licensed under the APACHE2.

We literally have hundreds of terraform modules that are Open Source and well-maintained. Check them out!

Usage

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"]
}

Subnet calculation logic

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.

${
  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 block data.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 of binary digits.

Example:

newbits = 1 - 2 subnets are available (1 binary digit allows to count up to 2)

newbits = 2 - 4 subnets are available (2 binary digits allows to count up to 4)

newbits = 3 - 8 subnets are available (3 binary digits allows to count up to 8)

etc.
1. We know, that we have 6 AZs in a us-east-1 region (see step 2).
2. We need to create 1 public subnet and 1 private subnet in each AZ, thus we need to create 12 subnets in total (6 AZs * (1 public + 1 private)).
3. We need 4 binary digits for that ( 2⁴ = 16 ). In order to calculate the number of binary digits we should use logarithm function. We should use base 2 logarithm because decimal numbers can be calculated as powers of binary number. See Wiki for more details
  
  Example:
  
  For 12 subnets we need 3.58 binary digits (log₂12)
  
  For 16 subnets we need 4 binary digits (log₂16)
  
  For 7 subnets we need 2.81 binary digits (log₂7)
  
  etc.
4. We can't use fractional values to calculate the number of binary digits. We can't round it down because smaller number of binary digits is insufficient to represent the required subnets. We round it up. See ceil.
  
  Example:
  
  For 12 subnets we need 4 binary digits (ceil(log₂12))
  
  For 16 subnets we need 4 binary digits (ceil(log₂16))
  
  For 7 subnets we need 3 binary digits (ceil(log₂7))
  
  etc.
5. Assign private subnets according to AZ number (we're using count.index for that).
6. Assign public subnets according to AZ number but with a shift according to the number of AZs in the region (see step 2)

Makefile Targets

Available targets:

  help                                Help screen
  help/all                            Display help for all targets
  help/short                          This help short screen
  lint                                Lint terraform code

Inputs

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
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
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

Outputs

Name	Description
private_route_table_ids
private_subnet_ids
public_route_table_ids
public_subnet_ids

Share the Love

Like this project? Please give it a ★ on our GitHub! (it helps us a lot)

Are you using this project or any of our other projects? Consider leaving a testimonial. =)

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.

Help

Got a question?

File a GitHub issue, send us an email or join our Slack Community.

Commercial Support

Work directly with our team of DevOps experts via email, slack, and video conferencing.

We provide commercial support for all of our Open Source projects. As a Dedicated Support customer, you have access to our team of subject matter experts at a fraction of the cost of a full-time engineer.

Questions. We'll use a Shared Slack channel between your team and ours.
Troubleshooting. We'll help you triage why things aren't working.
Code Reviews. We'll review your Pull Requests and provide constructive feedback.
Bug Fixes. We'll rapidly work to fix any bugs in our projects.
Build New Terraform Modules. We'll develop original modules to provision infrastructure.
Cloud Architecture. We'll assist with your cloud strategy and design.
Implementation. We'll provide hands-on support to implement our reference architectures.

Terraform Module Development

Are you interested in custom Terraform module development? Submit your inquiry using our form today and we'll get back to you ASAP.

Slack Community

Join our Open Source Community on Slack. It's FREE for everyone! Our "SweetOps" community is where you get to talk with others who share a similar vision for how to rollout and manage infrastructure. This is the best place to talk shop, ask questions, solicit feedback, and work together as a community to build totally sweet infrastructure.

Signup for our newsletter that covers everything on our technology radar. Receive updates on what we're up to on GitHub as well as awesome new projects we discover.

Contributing

Bug Reports & Feature Requests

Please use the issue tracker to report any bugs or file feature requests.

Developing

If you are interested in being a contributor and want to get involved in developing this project or help out with our other projects, we would love to hear from you! Shoot us an email.

In general, PRs are welcome. We follow the typical "fork-and-pull" Git workflow.

Fork the repo on GitHub
Clone the project to your own machine
Commit changes to your own branch
Push your work back up to your fork
Submit a Pull Request so that we can review your changes

NOTE: Be sure to merge the latest changes from "upstream" before making a pull request!

Copyright

License

See LICENSE for full details.

Licensed to the Apache Software Foundation (ASF) under one
or more contributor license agreements.  See the NOTICE file
distributed with this work for additional information
regarding copyright ownership.  The ASF licenses this file
to you under the Apache License, Version 2.0 (the
"License"); you may not use this file except in compliance
with the License.  You may obtain a copy of the License at

  https://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing,
software distributed under the License is distributed on an
"AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
KIND, either express or implied.  See the License for the
specific language governing permissions and limitations
under the License.