Connecting public subnets to the internet

When using IPv4, we will be connecting an IGW to the public subnet, which will allow us to assign public and Elastic IPs through 1:1 NAT to our instances. This means that any traffic coming into the public or Elastic IP will be directed by the IGW to the internal IP address of the instance (the IP address will be sourced from the subnet IP range). When using IPv6, the addresses are assigned directly to the instances in the public subnet, and by connecting an IGW, we allow the traffic to flow in and out of those instances.

The IGW is designed to be horizontally scaled, redundant, and highly available by default, so there is no need to configure any redundancy when attaching an IGW to our public subnet. 

To allow the traffic from the instances to flow to and from the internet, we will also need to create a default route in the routing table of each public subnet that has an IGW attached to it. For IPv4, we will need to create a route for 0.0.0.0/0, whereas a route for an IPv6 will have a destination of ::/0. Both routes will need to define the target as the ID of the IGW that is connected to the subnet. When creating a route in the AWS management console, simply start typing igw in the target for the default route and the IGW ID will pop up as a recommended value, as shown in the following screenshot: