How does a NAT Gateway function in AWS?

A NAT Gateway in AWS primarily facilitates outbound internet traffic from resources located in private subnets. When instances in a private subnet need to access the internet for updates, downloads, or accessing other services, they cannot do so directly due to the absence of a public IP address. The NAT Gateway mitigates this by allowing outbound requests from those private instances to the internet.

When an instance in a private subnet initiates a connection to the internet, the traffic is routed to the NAT Gateway, which has a public IP address. The NAT Gateway then translates the private IP address of the instance to its own public IP address, allowing the internet to respond to the request. When the response returns, the NAT Gateway translates the public IP back to the private IP, ensuring that the traffic reaches the appropriate instance.

This functionality is crucial for many scenarios where services or updates need to be fetched from the internet but the security policies require the instances to remain in private subnets, isolated from direct inbound traffic.