The answer is a misconfigured inbound rule on the EC2 instance’s security group, specifically failing to allow traffic from the ALB’s security group. Even though the scenario states the rule exists, the most likely cause of the ALB health check failing is that the security group rule is either referencing the wrong security group ID or is not properly associated with the instance, blocking the health check requests that originate from the ALB’s private IP addresses. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this tests your understanding of how security group references work across subnets—a common trap is assuming that a rule “allowing from the ALB’s security group” is automatically effective, when in fact the rule must be explicitly added to the target’s security group and the ALB’s security group ID must be correct. Remember the memory tip: “Health checks are just traffic—if the target’s SG doesn’t say yes, the ALB gets no response.”
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 has a VPC with a CIDR block of 172.16.0.0/16. They have two subnets: 172.16.1.0/24 (public) and 172.16.2.0/24 (private). They launch an EC2 instance in the private subnet and an Application Load Balancer (ALB) in the public subnet. The ALB needs to forward traffic to the EC2 instance on port 80. The security group for the EC2 instance allows inbound traffic on port 80 from the security group of the ALB. The ALB health checks are failing. What is the most likely cause?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The security group for the EC2 instance does not allow inbound traffic from the ALB's security group
Option D is correct because the EC2 instance's security group must have an inbound rule allowing traffic from the ALB's security group. If this rule is missing or misconfigured, the ALB's health check requests (which originate from the ALB's private IP addresses) will be blocked, causing health checks to fail. The scenario states the rule 'allows inbound traffic on port 80 from the security group of the ALB,' but if the rule is not properly associated or the ALB's security group ID is incorrect, traffic will be denied.
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.
✗
The ALB cannot route traffic to a private subnet
Why it's wrong here
ALB can route to any subnet in the VPC.
✗
The private subnet does not have a route to a NAT gateway
Why it's wrong here
Not needed for ALB to EC2 communication.
✗
The ALB is in a different VPC than the EC2 instance
Why it's wrong here
They are in the same VPC.
✓
The security group for the EC2 instance does not allow inbound traffic from the ALB's security group
Why this is correct
Health checks come from the ALB's security group, so it must be allowed.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often assume health check failures are due to routing or NAT issues, but the most common cause is a missing or misconfigured inbound security group rule on the target instance that does not explicitly allow traffic from the ALB's security group.
Detailed technical explanation
How to think about this question
Under the hood, ALB health checks are sent from the ALB's network interfaces (ENIs) in the public subnets using the source IP addresses of those ENIs. The security group on the EC2 instance must reference the ALB's security group (or allow inbound from the VPC CIDR) to permit these probes. A common subtlety is that the ALB's security group itself does not need an outbound rule for health checks because it implicitly allows outbound traffic, but the target's security group must explicitly allow inbound from the ALB's security group. In real-world scenarios, misconfigured security group references (e.g., using the wrong security group ID) are a frequent cause of health check failures.
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 company's IT admin needs to give a contractor read-only access to production logs without sharing account credentials. Using role-based access control (RBAC) and temporary scoped permissions — not a permanent shared password — is the correct pattern. Questions like this test whether you can apply least-privilege access across cloud identity services.
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: The security group for the EC2 instance does not allow inbound traffic from the ALB's security group — Option D is correct because the EC2 instance's security group must have an inbound rule allowing traffic from the ALB's security group. If this rule is missing or misconfigured, the ALB's health check requests (which originate from the ALB's private IP addresses) will be blocked, causing health checks to fail. The scenario states the rule 'allows inbound traffic on port 80 from the security group of the ALB,' but if the rule is not properly associated or the ALB's security group ID is incorrect, traffic will be denied.
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.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
About these practice questions
Courseiva creates original exam-style practice questions with explanations and wrong-answer analysis. It does not publish real exam questions, exam dumps, or protected exam content. Learn why practice questions differ from exam dumps →
Share a tip, memory trick, or ask about the reasoning behind this question. Do not post real exam questions, leaked content, braindumps, or copyrighted exam material. Comments are moderated and may be removed without notice.
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.
Question Discussion
Share a tip, memory trick, or ask about the reasoning behind this question. Do not post real exam questions, leaked content, braindumps, or copyrighted exam material. Comments are moderated and may be removed without notice.
Sign in to join the discussion.