When designing infrastructure in AWS, understanding the distinction between public and private subnets is fundamental to creating a secure and scalable architecture. Here’s a clear breakdown to help you differentiate between the two:
Public Subnets
A public subnet is one that is directly accessible from the internet. Instances in a public subnet can send and receive traffic from the internet via an Internet Gateway (IGW) attached to the VPC.
Key Characteristics:
Route Table: The public subnet's route table must have a route that directs all internet-bound traffic (0.0.0.0/0) to the Internet Gateway (IGW).
Use Case: These subnets are typically used for resources that need direct access to the internet, such as web servers, bastion hosts, or load balancers.
Access: Instances in public subnets are assigned public IPs (or Elastic IPs) to communicate directly with the internet.
Private Subnets
A private subnet is isolated from direct internet access. Instances in private subnets cannot send or receive traffic from the internet unless specific configurations are applied.
Key Characteristics:
Route Table: The private subnet’s route table does not include a direct route to the Internet Gateway (IGW).
Internet Access via NAT Gateway (NGW): While instances in private subnets don’t have public IPs, they can access the internet for outbound communication (e.g., downloading updates) via a NAT Gateway (NGW) or NAT Instance in a public subnet.
How to Differentiate Between Public and Private Subnets in AWS
You can differentiate between public and private subnets by checking their association with Internet Gateways (IGWs) or NAT Gateways (NGWs):
Public Subnet:
Route table includes a route to the IGW.
Instances have public IPs.
Private Subnet:
Route table does not include a direct route to the IGW.
May have a route to a NAT Gateway (NGW) for outbound internet access.
Understanding these differences is crucial for building secure and cost-effective solutions in AWS. If you're configuring subnets in your VPC, always design with your application requirements and security needs in mind!