Let's quickly go through some important concepts related to NAT instances:

• NAT instances are individual EC2 instances and we need to take care of patching, availability, and scaling. NAT gateways are maintained by AWS and AWS takes care of patching, availability, and scaling. 
• NAT instances can create bottlenecks if many EC2 instances are using them at the same time; that is, unless we spend extra effort to improve scaling manually.
• NAT instances are individual EC2 instances and are associated with security groups. NAT gateways are not associated with any security groups.
• It is a common practice to allocate Elastic IPs to the NAT instances.