ANS-C01 Network Design Practice Question
A company is designing a VPC for a three-tier web application. The web servers must be accessible from the internet, while the application and database servers must be isolated. Which subnet design should the company use?
⚠ Common exam trap
Test-takers frequently confuse network ACLs with security groups, thinking ACLs alone can isolate traffic within a single subnet, but ACLs cannot prevent instances in the same subnet from communicating with each other unless explicit deny rules are added, and they cannot replace the architectural separation of public and private subnets required for internet-facing vs. internal tiers.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Public subnet for web servers, private subnets for application and database servers
It uses a public subnet with an Internet Gateway (IGW) for the web servers to receive inbound traffic from the internet, while placing the application and database servers in private subnets with no direct internet route. This design enforces the principle of least privilege by isolating sensitive tiers, using NAT Gateway or VPC endpoints for outbound traffic if needed, and relying on security groups and network ACLs for fine-grained access control.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
A single subnet for all tiers with network ACLs to isolate traffic
Why it's wrong here
Single subnet does not provide physical isolation.
- ✓
Public subnet for web servers, private subnets for application and database servers
Why this is correct
Provides internet access to web servers while isolating backend tiers.
- ✗
Private subnets for all tiers
Why it's wrong here
Blocks internet access to web servers.
- ✗
Public subnets for all tiers
Why it's wrong here
Exposes backend servers to the internet.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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