Public vs private subnets in AWS - When to Use Which

The Big Idea

What if a simple network setup could protect your data and keep your app running smoothly without headaches?

The Scenario

Imagine setting up a network for your app where some parts need to be open to the internet, and others must stay hidden for security. Doing this by hand means manually configuring each server's access and firewall rules, which can get confusing fast.

The Problem

Manually managing which servers can talk to the internet or stay private is slow and risky. One wrong setting can expose sensitive data or block important traffic, causing downtime or security breaches.

The Solution

Using public and private subnets lets you organize your network clearly: public subnets for internet-facing resources, private subnets for secure internal ones. This setup automates access control and keeps your system safe and reliable.

Before vs After

✗ Before

Set firewall rules on each server manually to allow or block internet access.

✓ After

Create public and private subnets; assign servers accordingly; routing and security are handled automatically.

What It Enables

This approach makes it easy to build secure, scalable networks where sensitive parts stay protected while public parts stay reachable.

Real Life Example

A web app uses a public subnet for its web servers so users can connect, and a private subnet for its database servers to keep data safe from outside access.

Key Takeaways

Manual network setup is complex and error-prone.

Public and private subnets organize resources by access needs.

This improves security and simplifies management.

Practice

1. Which statement best describes a public subnet in AWS?

easy

A. It has direct internet access and assigns public IPs automatically.

B. It does not assign public IPs and blocks all internet traffic.

C. It is used only for internal communication within the VPC.

D. It requires a VPN connection to access the internet.

Solution

1. Step 1: Understand public subnet characteristics

   A public subnet allows resources to communicate directly with the internet by assigning public IP addresses.

2. Step 2: Compare with other subnet types

   Private subnets do not assign public IPs and restrict internet access, unlike public subnets.

3. Final Answer:

   It has direct internet access and assigns public IPs automatically. -> Option A

4. Quick Check:

   Public subnet = direct internet access [OK]

Hint: Public subnet = direct internet access + public IPs [OK]

Common Mistakes:

• Confusing private subnet with public subnet
• Thinking all subnets have internet access
• Assuming VPN is needed for public subnet

2. Which AWS subnet configuration correctly creates a private subnet?

easy

A. AssignPublicIpOnLaunch: true

B. EnableInternetGateway: true

C. MapPublicIpOnLaunch: false

D. RouteTable: 0.0.0.0/0 via IGW

Solution

1. Step 1: Identify private subnet IP assignment

   Private subnets do not assign public IPs automatically, so MapPublicIpOnLaunch must be false.

2. Step 2: Understand routing and gateway settings

   Internet Gateway (IGW) and routing to 0.0.0.0/0 are for public subnets, not private.

3. Final Answer:

   <code>MapPublicIpOnLaunch: false</code> -> Option C

4. Quick Check:

   Private subnet = no public IP assignment [OK]

Hint: Private subnet disables public IP assignment [OK]

Common Mistakes:

• Setting public IP assignment to true for private subnet
• Confusing internet gateway with subnet config
• Using public route table for private subnet

3. Given this route table for a subnet:
Destination: 0.0.0.0/0, Target: igw-12345
What type of subnet is this likely to be?

medium

A. Public subnet with internet access

B. Private subnet with no internet access

C. Isolated subnet with VPN only

D. Subnet with NAT gateway only

Solution

1. Step 1: Analyze route table destination and target

   The route directs all internet traffic (0.0.0.0/0) to an internet gateway (igw), which enables internet access.

2. Step 2: Determine subnet type from routing

   Routing to an internet gateway means the subnet is public and can access the internet directly.

3. Final Answer:

   Public subnet with internet access -> Option A

4. Quick Check:

   Route to IGW = public subnet [OK]

Hint: Route 0.0.0.0/0 to IGW means public subnet [OK]

Common Mistakes:

• Assuming NAT gateway means public subnet
• Confusing IGW with VPN or NAT
• Ignoring route table targets

4. You created a subnet with MapPublicIpOnLaunch: false but instances still get public IPs. What is the likely issue?

medium

A. The subnet is associated with a route table that routes to an internet gateway.

B. The instance launch configuration overrides subnet settings to assign public IPs.

C. The VPC has no internet gateway attached.

D. The subnet CIDR block overlaps with another subnet.

Solution

1. Step 1: Understand subnet vs instance IP assignment

   Subnet setting disables automatic public IP assignment, but instance launch settings can override this.

2. Step 2: Identify cause of public IP despite subnet config

   If instance launch config explicitly assigns public IPs, it overrides subnet default behavior.

3. Final Answer:

   The instance launch configuration overrides subnet settings to assign public IPs. -> Option B

4. Quick Check:

   Instance config can override subnet IP assignment [OK]

Hint: Instance launch config can override subnet IP settings [OK]

Common Mistakes:

• Blaming route table for IP assignment
• Ignoring instance-level settings
• Confusing CIDR overlap with IP assignment

5. You want to host a web server accessible from the internet and a database only accessible internally. How should you design your subnets?

hard

A. Place both web server and database in private subnets and use a VPN for access.

B. Place both web server and database in a public subnet with security groups restricting access.

C. Place the web server in a private subnet and the database in a public subnet.

D. Place the web server in a public subnet and the database in a private subnet.

Solution

1. Step 1: Identify subnet roles for internet access

   The web server needs internet access, so it belongs in a public subnet with a route to the internet gateway.

2. Step 2: Secure the database internally

   The database should be in a private subnet without direct internet access to keep it secure.

3. Final Answer:

   Place the web server in a public subnet and the database in a private subnet. -> Option D

4. Quick Check:

   Internet-facing resources in public, internal in private [OK]

Hint: Web server public, database private subnet [OK]

Common Mistakes:

• Putting database in public subnet
• Putting web server in private subnet
• Relying only on security groups without subnet design