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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.

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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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Copyright

Copyright © 2017-2018 Cloud Posse, LLC

License

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.

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Contributors

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

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Terraform module for public and private subnets provisioning in existing VPC

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