A private subnet is a subnet within a Virtual Private Cloud (VPC) that does not have direct access to the internet. This isolation is achieved through its route table configuration.
Key Characteristics of a Private Subnet
The defining characteristic of a private subnet lies in its route table configuration.
- No Internet Gateway Route: A private subnet's route table lacks a route to an Internet Gateway. According to the provided reference, "a private subnet is a subnet that is associated with a route table that doesn't have a route to an internet gateway." This absence prevents instances in the subnet from directly communicating with the public internet.
Comparison: Public Subnet vs. Private Subnet
To better understand private subnets, it's helpful to contrast them with public subnets:
| Feature | Public Subnet | Private Subnet |
|---|---|---|
| Internet Access | Direct access to the internet. | No direct access to the internet. |
| Route Table | Contains a route to an Internet Gateway. | Does not contain a route to an Internet Gateway. |
| Use Cases | Hosting public-facing applications (e.g., web servers). | Hosting backend applications, databases, etc. |
How Private Subnets Communicate Outbound
While private subnets don't have direct internet access, they often need to communicate with external resources for updates, patches, or accessing AWS services. This is typically achieved through:
- NAT Gateway: A NAT (Network Address Translation) gateway allows instances in the private subnet to initiate outbound traffic to the internet, but prevents inbound traffic initiated from the internet.
- NAT Instance: Similar to a NAT gateway, but implemented using an EC2 instance. NAT gateways are generally preferred for their higher availability and scalability.
- VPC Endpoints: For accessing AWS services (e.g., S3, DynamoDB) without traversing the public internet, you can use VPC endpoints. These endpoints create a private connection between your VPC and the AWS service.
Security Benefits of Private Subnets
- Enhanced Security: By isolating resources from the internet, private subnets significantly reduce the attack surface and minimize the risk of unauthorized access.
- Data Protection: Sensitive data, such as database credentials and user information, can be stored in private subnets, protecting it from direct internet exposure.
- Compliance: Private subnets help organizations meet compliance requirements that mandate the isolation of certain data and applications.
