What is the difference between public and private subnets within a VPC?

Public and private subnets within a Virtual Private Cloud (VPC) serve distinct purposes based on their connectivity to the internet.

Public subnets are designed to allow resources within them, such as EC2 instances, to have direct access to the internet. This is typically achieved by routing the traffic through an Internet Gateway. This means that resources in a public subnet can communicate freely with external services and can also be accessed from the internet, making them suitable for hosting public-facing applications.

On the other hand, private subnets do not have direct access to the internet. Resources in these subnets cannot communicate directly with the outside world, which makes them ideal for hosting sensitive applications or databases that should not be exposed to the public internet. If these resources need to access the internet (for example, for software updates), this is typically done through a Network Address Translation (NAT) gateway or instance located in a public subnet.

This distinction in connectivity is fundamental when architecting applications in a VPC, determining how and where resources can be accessed based on their security requirements and use cases.