After adding a NAT instance to a VPC, what is a necessary step to ensure instances in a private subnet can access the internet?

To ensure that instances in a private subnet can access the internet through a NAT instance, it's necessary to disable the source/destination checks on the NAT instance. This step allows the NAT instance to function properly as a router.

When a NAT instance is set up, it needs to be able to route traffic between the instances in the private subnet (which do not have direct internet access) and the internet. By default, Amazon EC2 instances have source/destination checks enabled, which means they can only send traffic to or receive traffic from a specific source. Disabling these checks allows the NAT instance to route the traffic from the private instances to the internet and back. This is crucial because the traffic originating from private instances does not have direct internet access; it goes through the NAT instance, which handles the necessary translations and forwarding.

In addition to this step, making sure that the NAT instance has an Elastic IP assigned ensures that it has a public IP address to communicate with the internet. However, the key action specifically related to configuring the NAT instance's ability to route traffic from private subnets is the disabling of source/destination checks. This setup is essential for the NAT instance to perform its intended function effectively.