What is the difference between public and private subnets?

In an AWS VPC, the key difference between a public subnet and a private subnet lies in their access to the internet and the security configurations applied to each.

Public Subnet:

•   Internet Access:
  

    A public subnet is one that has direct access to the internet. Instances within a public subnet can communicate directly with the internet via an Internet Gateway (IGW) attached to the VPC. This is typically used for resources that need to be publicly accessible, such as web servers or load balancers.

•   Routing:
  

    The route table for a public subnet includes a route directing traffic destined for the internet (0.0.0.0/0) to the Internet Gateway. This allows instances to send and receive traffic from the internet.

•   Use Cases:
  

    Public subnets are ideal for hosting publicly accessible services, such as web applications or API endpoints, that need to be accessed from the outside world.

Private Subnet:

•   No Direct Internet Access:
  

    A private subnet, on the other hand, is isolated from the internet. Instances within a private subnet cannot access the internet directly. If they need internet access (e.g., to download updates), they must route through a NAT Gateway or NAT instance located in a public subnet.

•   Routing:
  

    The route table for a private subnet typically routes traffic destined for the internet through a NAT Gateway in a public subnet, ensuring that the private instances can initiate outbound connections but do not accept inbound traffic from the internet.

•   Use Cases:
  

    Private subnets are commonly used for backend resources such as databases, application servers, and internal services that do not need to be publicly accessible but still require secure connectivity with the internet.

In summary, public subnets allow direct internet access for instances, while private subnets keep instances isolated from inbound internet traffic, providing more security and control over network interactions.