Public vs private subnets in AWS - Quick Revision & Key Differences

Recall & Review

beginner

What is a public subnet in AWS?

A public subnet is a subnet that has a route to the internet through an internet gateway. Resources in this subnet can communicate directly with the internet.

beginner

What is a private subnet in AWS?

A private subnet is a subnet that does not have a direct route to the internet. Resources here cannot be accessed directly from the internet but can access the internet through a NAT device.

intermediate

Why use private subnets instead of public subnets?

Private subnets increase security by keeping sensitive resources away from direct internet access, reducing exposure to attacks.

intermediate

How does a NAT gateway help private subnets?

A NAT gateway allows resources in private subnets to access the internet for updates or downloads without exposing them to inbound internet traffic.

beginner

What AWS component enables internet access for a public subnet?

An Internet Gateway attached to the VPC enables internet access for public subnets by routing traffic between the subnet and the internet.

Which AWS subnet type has a direct route to the internet?

A VPN subnet

B Private subnet

C Isolated subnet

D Public subnet

Public subnets have a route to the internet via an internet gateway.

What device allows private subnet resources to access the internet safely?

A NAT Gateway

B Load Balancer

C VPN Gateway

D Internet Gateway

NAT Gateway enables outbound internet access for private subnet resources without exposing them to inbound internet traffic.

Which subnet type is best for hosting a public web server?

A Private subnet

B Isolated subnet

C Public subnet

D Backend subnet

Public subnets allow direct internet access, suitable for public web servers.

What is a key security benefit of private subnets?

A No inbound internet traffic

B No internet access at all

C Direct internet access

D Automatic backups

Private subnets block inbound internet traffic, reducing exposure to attacks.

Which AWS component must be attached to a VPC to enable public subnet internet access?

A NAT Gateway

B Internet Gateway

C Virtual Private Gateway

D Transit Gateway

Internet Gateway connects the VPC to the internet, enabling public subnet access.

Explain the difference between a public subnet and a private subnet in AWS.

Think about how resources connect to the internet and security.

Describe how a NAT gateway works with private subnets to allow internet access.

Focus on the path outbound traffic takes from private subnet.

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