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.

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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)
}

1. 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)

2. Get number of available AZ in the region (e.g. length(data.aws_availability_zones.available.names))

3. 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

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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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Copyright

Copyright © 2017-2018 Cloud Posse, LLC

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
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Contributors

Erik Osterman	Andriy Knysh	Sergey Vasilyev	Vladimir	Konstantin B	dcowan-vestmark	Ivan Pinatti