The correct answer is to configure the ALB security group to allow inbound traffic from the on-premises CIDR range. This works because an Application Load Balancer must reside in a public subnet to be reachable from an on-premises network via an AWS Site-to-Site VPN; the VPN terminates on a virtual private gateway or transit gateway, but the ALB itself requires a public subnet with a route to the internet gateway or virtual private gateway to accept inbound traffic from outside the VPC. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this scenario tests your understanding of how ALB routing interacts with VPN connectivity, often appearing as a trap where candidates mistakenly think a private subnet suffices for VPN traffic. A common memory tip is “ALB needs a public face, even for VPN grace”—the load balancer must be internet-facing or placed in a public subnet with the correct security group rules, regardless of the traffic source.
ANS-C01 Network Design Practice Question
This ANS-C01 practice question tests your understanding of network design. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
A company is deploying a new application in a VPC. The application consists of EC2 instances in an Auto Scaling group behind an Application Load Balancer (ALB). The ALB must only receive traffic from the company's on-premises network via an AWS Site-to-Site VPN. Which THREE steps should the network engineer take to meet this requirement?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Place the ALB in a public subnet.
Option B is correct because an Application Load Balancer (ALB) must be placed in a public subnet to be reachable from an on-premises network via an AWS Site-to-Site VPN. The VPN connection terminates on a virtual private gateway or transit gateway, which routes traffic to the VPC, but the ALB itself needs a public subnet with a route to the internet gateway (or virtual private gateway) to accept inbound traffic from the VPN. Without a public subnet, the ALB cannot receive traffic from outside the VPC, including VPN traffic.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
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 ALB in a private subnet and use a Transit Gateway.
Common exam trap: answer the scenario, not the keyword
AWS often tests the misconception that an ALB in a private subnet can receive traffic from a VPN via a Transit Gateway or NAT gateway, but the ALB must be in a public subnet to have a route to the virtual private gateway for inbound traffic from on-premises.
Detailed technical explanation
How to think about this question
An ALB in a public subnet requires a route table entry pointing to the internet gateway (or virtual private gateway for VPN traffic) for inbound traffic. The ALB's security group and network ACL must explicitly allow the on-premises CIDR range, as the VPN tunnel encapsulates traffic with the on-premises source IP. Under the hood, the ALB's elastic network interface (ENI) in the public subnet uses the VPC's route table to direct return traffic back through the VPN, ensuring symmetric routing.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
TExam Day Tips
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A healthcare organisation deploys an application with a public-facing web tier and a private database tier. The database subnet has no public IP and only accepts connections from the web tier's security group. Questions like this test whether you can design cloud network isolation using VNets/VPCs, subnets, and security group rules.
What to study next
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Network Design — This question tests Network Design — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Place the ALB in a public subnet. — Option B is correct because an Application Load Balancer (ALB) must be placed in a public subnet to be reachable from an on-premises network via an AWS Site-to-Site VPN. The VPN connection terminates on a virtual private gateway or transit gateway, which routes traffic to the VPC, but the ALB itself needs a public subnet with a route to the internet gateway (or virtual private gateway) to accept inbound traffic from the VPN. Without a public subnet, the ALB cannot receive traffic from outside the VPC, including VPN traffic.
What should I do if I get this ANS-C01 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Question Discussion
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