terraform.aws-baseline-infra/modules/networking/vpc_subnets/README.md

# Overview

This module performs the following tasks:

- Create VPC, vpcflow log
- Create subnets in every AZ
- Create IGW, NGW

## Subnet addressing

Subnet cidrs are calculated automatically. Due to the design of terraform's cidrsubnets, this module has limitations: 

* supports 2, 4, 6, or 8 subnets in total. 
* hard-coded to work with 2 AZs, regardless of number of AZs available in the region. 

Based on the input variables, it will create subnet cidrs using the following function

| Private Subnets per az | Public Subnets per az | Function                                     | Example if a /24 is used on VPC |
| ---------------------- | --------------------- | -------------------------------------------- | ------------------------------- |
| 1                      | 0                     | cidrsubnets(local.vpc-cidr, 1,1)             | 2 * /25                         |
| 1                      | 1                     | cidrsubnets(local.vpc-cidr, 2,2,2,2)         | 4 * /26                         |
| 2                      | 1                     | cidrsubnets(local.vpc-cidr, 3,3,3,3,3,3)     | 6 * /27                         |
| 2                      | 2                     | cidrsubnets(local.vpc-cidr, 4,4,4,4,4,4,4,4) | 8 * /28                         |

simple-divide = local.total-no-subnets >=8 ? cidrsubnets(local.vpc-cidr, 4,4,4,4,4,4,4,4) : local.total-no-subnets >=6 ? cidrsubnets(local.vpc-cidr, 3,3,3,3,3,3) : local.total-no-subnets >=4 ? cidrsubnets(local.vpc-cidr, 2,2,2,2) : local.total-no-subnets >=2 ? cidrsubnets(local.vpc-cidr, 1,1) : null

## Inputs:

| Name                             | Description                                       | Type   | Default | Required |
| -------------------------------- | ------------------------------------------------- | ------ | ------- |:--------:|
| application                      | name of application                               | string | none    | yes      |
| environment                      | capacity of environment (prd/dev/lab)             | string | none    | yes      |
| customer-name                    | owner of aws resources                            | string | none    | yes      |
| project                          | name of project                                   | string | none    | yes      |
| default-tags                     | tags to be added to resources                     | list   | none    | yes      |
| number-of-private-subnets-per-az | number of private subnets per az                  | number | 0       | yes      |
| number-of-public-subnets-per-az  | number of public subnets per az                   | number | 0       | yes      |
| create-nat-gateway               | whether to deploy NAT gateway for private subnets | bool   | true    | yes      |
| vpc-cidr                         | VPC cidr                                          | string | none    | yes      |
| enable-flowlog                   | whether to enable vpc flowlog                     | bool   | true    | yes      |
| vpcflowlog-retain-days           | number of days to retain vpc cloudwatch log       | number | 90      | yes      |
| aws-region-short                 | short name of aws region (e.g. apne1)             | string | none    | yes      |
| aws-region                       | aws region (e.g. ap-northeast-1)                  | string | none    | yes      |
| vpcflowlog-cwl-loggroup-key-arn  | kms key alias arn for log group encryption        | string | none    | yes      |

## Outputs:

| Name            | Description         | Type   |
| --------------- | ------------------- | ------ |
| vpc_id          | vpc id              | string |
| public_subnets  | list of cidr blocks | list   |
| private_subnets | list of cidr blocks | list   |