Subnets

Subnets

Subnets reside inside a VPC, logically separating your services. Subnets exist in Availability Zones (AZ)s, such as us-west-2a or us-west-2b. The idea is that one AZ might go down, but if your system is spanned across multiple, your service will stay up. This means you’ll need to configure multiple subnets, usually two, for each service.

This typically means to make your service highly available, you’ll need to run two or more instances. (TODO: mention application load balancers across AZs).

Creating a Subnet in the default VPC

In the AWS portal, search for Subnet and click the one that lists itself as a VPC feature. You’ll be presented with a list of your current subnets. Click the Create subnet button at the top right.

In the VPC ID dropdown, select the default VPC (or whichever you’d prefer).

Fill out Subnet name which is useful to name after the app that you’ll be hosting in it. Select an Availability zone and remember not to use the same AZ for your second subnet. Choose the IPv4 VPC CIDR block where you want this subnet to live, then create a new IPv4 subnet CIDR block.

Sizing is highly dependent on the application needs but I recommend starting with /24, as it keeps your numbers easy to deal with, and each VPC comes with a huge amount of space by default.

Click Create subnet.

Example

To create matching subnets in multiple AZs, my subnets might look like

- name: myapp-2a
  AZ: us-west-2a
  IPv4 VPC CIDR block: 10.0.0.0/16
  IPv4 subnet CIDR block: 10.0.1.0/24
- name: myapp-2b
  AZ: us-west-2b
  IPv4 VPC CIDR block: 10.0.0.0/16
  IPv4 subnet CIDR block: 10.0.2.0/24

Terraform

You’ll need to provide the VPC ID either in the code or via the TF_VAR_vpc_id environment variable.

variable "vpc_id" {}

data "aws_vpc" "selected" {
  id = var.vpc_id
}

resource "aws_subnet" "myapp2a" {
  vpc_id     = aws_vpc.selected.id
  cidr_block = "10.0.1.0/24"
  availability_zone = "us-west-2a"

  tags = {
    Name = "myapp-2a"
  }
}

resource "aws_subnet" "myapp2b" {
  vpc_id     = aws_vpc.selected.id
  cidr_block = "10.0.2.0/24"
  availability_zone = "us-west-2b"

  tags = {
    Name = "myapp-2b"
  }
}