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A NAT instance is created in one subnet, meaning one AZ where the subnet is created. If you have resources in multiple AZs and in case the AZ that hosts the NAT Gateway is down, resources hosted in the other AZs lose the internet connection. So to have a highly available NAT infrastructure, you need to create a NAT Gateway which is a managed NAT service with a NAT instance created in multiple AZs to have fault tolerance.