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A Virtual Private Cloud (VPC) spans an entire AWS Region. However, you cannot launch an EC2 instance directly into a VPC. You must first divide your VPC into smaller network chunks called Subnets.
Unlike a VPC, a Subnet is tied to a single Availability Zone (AZ).
When you create a VPC, you assign it an IPv4 CIDR block (e.g., 10.0.0.0/16), which provides 65,536 IP addresses.
When you create a Subnet, you assign it a smaller CIDR block that falls within the VPC's range (e.g., 10.0.1.0/24), which provides 256 IP addresses.
Note: AWS reserves the first four and the last IP address in every subnet for internal networking purposes, so a /24 subnet actually gives you 251 usable IPs.
There is no technical setting or checkbox that makes a subnet "Public" or "Private" when you create it. The difference is entirely determined by Route Tables.
A subnet is considered Public if its associated Route Table has a route that sends traffic destined for the internet (0.0.0.0/0) to an Internet Gateway (IGW).
If you launch a web server here and assign it a Public IP, anyone on the internet can access it.
A subnet is Private if its Route Table does not have a route to an Internet Gateway.
However, servers in a private subnet often need to download software updates from the internet. To allow outbound-only internet access, you route 0.0.0.0/0 traffic to a NAT Gateway (which must reside in the Public Subnet).
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