Public vs Private Subnets: Complete Guide to Network Architecture and Security

Understanding the difference between public and private subnets is fundamental to designing secure, scalable cloud infrastructure. These networking concepts form the backbone of modern cloud architectures, determining how your resources communicate with the internet and with each other. Whether you're building a simple web application or a complex microservices architecture, mastering subnet design is essential for creating robust, secure systems.

The key difference between public and private subnets lies in internet accessibility. Public subnets can directly communicate with the internet, while private subnets cannot without going through a NAT Gateway or similar service.

✅ Answer

Type	Internet Access	Use Case	Route Table Configuration
Public Subnet	Yes (via Internet Gateway)	Load balancers, Bastion hosts, public APIs	Route to Internet Gateway (IGW)
Private Subnet	No direct internet access	Databases, app servers, internal services	No direct route to IGW; NAT Gateway optional

📘 Detailed Explanation

🌐 Public Subnet

A public subnet is a subnet that has a route to an Internet Gateway (IGW).
EC2 instances in this subnet can be accessed from the internet if they have public IPs.
Common use cases:
- Web servers
- Bastion hosts
- NAT Gateways

📌 Route Table Example:

Destination      Target
0.0.0.0/0        igw-xxxxxxxx

🔒 Private Subnet

A private subnet has no direct route to the internet.
Instances cannot be accessed directly from the internet even if they have a public IP (which they shouldn’t).
If outbound access is needed (e.g., to install packages), they route traffic via a NAT Gateway placed in a public subnet.

📌 Route Table Example (with NAT):

Destination      Target
0.0.0.0/0        nat-xxxxxxxx

🧠 Analogy

Public Subnet: Like a house with a door that opens directly to the street (internet).
Private Subnet: Like a room in a gated community — you can go out, but only through controlled paths (NAT).

Summary:

Public subnet → has internet access via IGW.

Private subnet → no direct internet access. Used for backend services, databases, and sensitive components.