What is the difference between a public and private subnet in a Amazon VPC?

Solution 1:

The main difference is the route for 0.0.0.0/0 in the associated route table.

A private subnet sets that route to a NAT instance. Private subnet instances only need a private ip and internet traffic is routed through the NAT in the public subnet. You could also have no route to 0.0.0.0/0 to make it a truly private subnet with no internet access in or out.

A public subnet routes 0.0.0.0/0 through an Internet Gateway (igw). Instances in a public subnet require public IPs to talk to the internet.

The warning appears even for private subnets, but the instance is only accessible inside your vpc.

Solution 2:

As documented here

PUBLIC SUBNET If a subnet's traffic is routed to an internet gateway, the subnet is known as a public subnet. PRIVATE SUBNETIf a subnet doesn't have a route to the internet gateway, the subnet is known as a private subnet.