More detail
A subnet is public when its route table sends internet traffic (0.0.0.0/0) to an internet gateway. Otherwise it’s private. All the subnets in the default VPC are public.
Subnet
A slice of a VPC's addresses that lives in one Availability Zone.
Think of it as: One street on the estate.
What to know for Level 0
- A subnet is public only if its route table sends
0.0.0.0/0to an internet gateway. Public IPs and security groups don't make a subnet public. - A subnet lives in exactly one Availability Zone, so surviving an AZ failure needs subnets in more than one.
- Databases go in private subnets, with no route to the internet gateway, and a security group that only lets the app in.