# Overview
This module performs the following tasks:

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

## Subnet addressing
This module takes in the VPC cidr. Then add 4 bits to the netmask and divide the cidr into 2 ranges.
First range will be used for private subnet and second for public subnets. 
Another 4 bits are added to these ranges for each subnet.

For example, if the VPC cidr is 10.2.0.0/16, the following subnets will be created:

| Subnet Type | Subnet AZ1 | Subnet AZ2 | Subnet AZ3 |
|-------------|------------|------------|------------|
| Private  | 10.2.0.0/24 | 10.2.1.0/24 | 10.2.2.0/24 |
| Public   | 10.2.16.0/24 | 10.2.17.0/24 | 10.2.18.0/24 |

The VPC cidr netmask should be /20 or above, to produce subnets with /28 netmasks or above.
Subnet smaller than /28 is unlikely useful. 

## 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 |