Public vs private subnets in AWS - Practice Questions

Challenge - 5 Problems

🧠 Conceptual

intermediate

Understanding subnet accessibility

Which statement correctly describes the main difference between a public subnet and a private subnet in AWS?

A A public subnet has a route to an internet gateway, allowing direct internet access; a private subnet does not have this route.

B A private subnet has a route to an internet gateway, allowing direct internet access; a public subnet does not have this route.

C Both public and private subnets have direct internet access but differ in security group settings.

D Public subnets are only used for databases, while private subnets are used for web servers.

💡 Hint

Think about which subnet can send traffic directly to the internet.

Public subnets have a route to the internet gateway, enabling resources to communicate directly with the internet. Private subnets lack this route, so their resources cannot access the internet directly.

❓ Architecture

intermediate

Subnet design for a web application

You are designing a web application architecture in AWS. Which subnet setup is best practice for hosting web servers and databases?

A Place web servers in a private subnet and databases in a public subnet.

B Place both web servers and databases in a public subnet.

C Place web servers in a public subnet and databases in a private subnet.

D Place both web servers and databases in a private subnet.

💡 Hint

Consider which resources need internet access and which should be protected.

Web servers need internet access to serve users, so they go in public subnets. Databases should be protected from direct internet access, so they go in private subnets.

❓ security

advanced

Security implications of subnet routing

What is the security risk if a private subnet mistakenly has a route to an internet gateway?

A There is no risk; routing to an internet gateway does not affect security.

B Resources in the private subnet become directly accessible from the internet, increasing exposure to attacks.

C The subnet automatically becomes public and AWS charges extra fees.

D Resources in the private subnet lose all internet access, causing application failures.

💡 Hint

Think about what happens when private resources can be reached from outside.

If a private subnet has a route to an internet gateway, its resources can be accessed directly from the internet, which can expose sensitive data and increase attack surface.

❓ service_behavior

advanced

NAT Gateway role in private subnet internet access

How does a NAT Gateway enable instances in a private subnet to access the internet?

A It converts private IP addresses to IPv6 addresses for internet access.

B It assigns public IP addresses directly to instances in the private subnet.

C It blocks all internet traffic to maintain subnet privacy.

D It allows outbound internet traffic from private subnet instances by routing through a public subnet with an internet gateway.

💡 Hint

Consider how private subnet instances send requests to the internet without direct access.

A NAT Gateway in a public subnet routes outbound traffic from private subnet instances to the internet, allowing them to access external services without exposing their private IPs.

✅ Best Practice

expert

Subnet IP addressing and CIDR block planning

You have a VPC with CIDR block 10.0.0.0/16. You want to create two public subnets and two private subnets with equal size and no overlap. Which CIDR blocks correctly divide the VPC into these four subnets?

A 10.0.0.0/18, 10.0.64.0/18 for public subnets; 10.0.128.0/18, 10.0.192.0/18 for private subnets.

B 10.0.0.0/17, 10.0.128.0/17 for public subnets; 10.0.64.0/17, 10.0.192.0/17 for private subnets.

C 10.0.0.0/19, 10.0.32.0/19 for public subnets; 10.0.64.0/19, 10.0.96.0/19 for private subnets.

D 10.0.0.0/16 for public subnets; 10.0.0.0/16 for private subnets.

💡 Hint

Remember that /16 means 65536 IPs. Dividing into four equal subnets means each subnet has /18 (16384 IPs).

Dividing a /16 block into four /18 blocks creates four equal subnets without overlap. Option A correctly assigns four /18 CIDR blocks covering the entire 10.0.0.0/16 range.