ANS-C01 Network Implementation Practice Question
A company is deploying a multi-tier web application on AWS. The web tier runs on EC2 instances behind an Application Load Balancer (ALB), and the application tier runs on EC2 instances that connect to an RDS MySQL Multi-AZ DB instance. The application tier must be isolated from the internet and only accessible from the web tier. Which network implementation meets these requirements with the LEAST administrative overhead?
⚠ Common exam trap
The trap here is that candidates often over-engineer the solution by adding unnecessary components like NAT gateways or VPC peering, misunderstanding that security group references can securely allow traffic from a public subnet to a private subnet within the same VPC without internet gateway or NAT involvement.
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
✓
Place the web tier in a public subnet and the application tier in a private subnet, and configure security groups to allow inbound traffic to the application tier only from the web tier's security group.
Placing the web tier in a public subnet and the application tier in a private subnet, then using a security group rule that references the web tier's security group as the source, provides a stateful, application-layer firewall that allows traffic only from the web tier to the application tier. This design isolates the application tier from the internet while requiring no additional infrastructure like NAT gateways or VPC peering, minimizing administrative overhead. The security group reference automatically scales with the web tier's instances, eliminating the need to manage IP addresses.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
Place the web tier in a public subnet and the application tier in a private subnet, and configure security groups to allow inbound traffic to the application tier only from the web tier's security group.
Why this is correct
This achieves isolation with minimal overhead.
- ✗
Create two VPCs: one for the web tier with a public subnet, and one for the application tier with a private subnet, and connect them using VPC peering.
Why it's wrong here
Creating two VPCs with peering introduces cross-VPC routing and separate CIDR management, which adds administrative overhead compared to a single VPC with private subnets and a NAT gateway. This option is tempting because VPC peering is designed for securely connecting separate VPCs, and would be correct if the application tier required isolation from the web tier's VPC for compliance or multi-account boundaries, but the stem demands least overhead within one VPC.
- ✗
Place the web tier in a public subnet and the application tier in a private subnet with a NAT gateway for outbound access, and use security groups to allow traffic from the web tier.
Why it's wrong here
NAT gateway is unnecessary and adds cost; no outbound access required.
- ✗
Place both tiers in the same public subnet, and use a network ACL to restrict traffic from the web tier to the application tier.
Why it's wrong here
Public subnets have direct internet access, violating isolation.
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Same concept, more angles
2 more ways this is tested on ANS-C01
These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. A company is running a two-tier application with a web tier and a database tier. The web tier must be accessible from the internet, but the database tier should only be accessible from the web tier. Which architecture should they use?
hard- ✓ A.Web tier in public subnet, database tier in private subnet with a security group allowing only web tier.
- B.Both tiers in public subnets with security groups restricting database access.
- C.Both tiers in private subnets with a NAT Gateway for internet access.
- D.Web tier in private subnet, database tier in public subnet with a security group.
Why A: The correct architecture places the web tier in a public subnet with an internet gateway for inbound traffic, and the database tier in a private subnet with a security group that only allows inbound traffic from the web tier's security group. This ensures the database is not directly reachable from the internet, adhering to the principle of least privilege and defense in depth. The security group acts as a stateful virtual firewall, allowing you to reference the web tier's security group as the source, which automatically adapts to any changes in the web tier's IP addresses.
Variation 2. A company has a multi-tier application deployed in a VPC. The web tier consists of an internet-facing Application Load Balancer (ALB) in public subnets, and EC2 instances in private subnets. The application tier runs on EC2 instances in separate private subnets, and the database tier uses an Amazon RDS for MySQL instance in private subnets. The application tier needs to connect to the database on port 3306. The security group for the RDS instance (sg-database) has an inbound rule allowing TCP 3306 from the security group of the application tier (sg-app). The application tier instances can connect to the database, but the web tier instances cannot. The web tier instances should not have direct database access. What is the most likely reason for the web tier's inability to connect to the database?
hard- A.The network ACL for the database subnet is blocking inbound traffic from the web tier subnets.
- ✓ B.The security group of the RDS instance does not have an inbound rule allowing traffic from the web tier's security group.
- C.The RDS instance is in a public subnet and requires a NAT Gateway for communication.
- D.The route table for the web tier subnets does not have a route to the database subnets.
Why B: The security group for the RDS instance (sg-database) only allows inbound traffic on port 3306 from the application tier's security group (sg-app). The web tier instances are in a different security group, so traffic from the web tier is implicitly denied. Option A is incorrect because network ACLs are stateless; if they were blocking web tier traffic, they would also affect the application tier, which can connect successfully. Option C is incorrect because the RDS instance is in private subnets and does not need a NAT Gateway; NAT Gateways are for outbound internet access. Option D is incorrect because route tables do not affect security group logic; routing only determines network path, not whether traffic is permitted by security groups.
JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This ANS-C01 practice question is part of Courseiva's free Amazon Web Services certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the ANS-C01 exam.