- A
The ALB cannot route traffic to a private subnet
Correct. The ALB requires targets to be in subnets with a route to an internet gateway. Since the EC2 instance is in a private subnet, the ALB cannot forward health check requests to it.
- B
The private subnet does not have a route to a NAT gateway
Why wrong: Incorrect. A NAT gateway is used for outbound internet access from private subnets and is not required for ALB health checks.
- C
The ALB is in a different VPC than the EC2 instance
Why wrong: Incorrect. The stem states that the ALB and EC2 instance are in the same VPC.
- D
The security group for the EC2 instance does not allow inbound traffic from the ALB's security group
Why wrong: Incorrect. The stem explicitly states that the security group allows inbound traffic on port 80 from the ALB's security group, so a security group misconfiguration is not the issue.
ANS-C01 Application Load Balancer (ALB) Health Checks 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. A key principle to apply: application Load Balancer (ALB) Health Checks. 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 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
The ALB cannot route traffic to a private subnet
The most likely cause is that the Application Load Balancer cannot route traffic to an instance in a private subnet. By default, an ALB can only target instances in public subnets that have a route to an internet gateway. The private subnet lacks such a route, so the ALB cannot reach the EC2 instance for health checks, causing them to fail.
Key principle: Application Load Balancer (ALB) Health Checks
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 this is correct
Correct. The ALB requires targets to be in subnets with a route to an internet gateway. Since the EC2 instance is in a private subnet, the ALB cannot forward health check requests to it.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Application Load Balancer (ALB) Health Checks
- ✗
The private subnet does not have a route to a NAT gateway
- ✗
The ALB is in a different VPC than the EC2 instance
Why it's wrong here
Incorrect. The stem states that the ALB and EC2 instance are in the same VPC.
- ✗
The security group for the EC2 instance does not allow inbound traffic from the ALB's security group
Why it's wrong here
Incorrect. The stem explicitly states that the security group allows inbound traffic on port 80 from the ALB's security group, so a security group misconfiguration is not the issue.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap is that candidates often think the security group rule is correctly configured because the stem says it 'allows inbound traffic', but they overlook that the rule must be explicitly added and correctly reference the ALB's security group ID. Even a minor misconfiguration (like a typo in the security group ID) will cause health checks to fail.
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
- Application Load Balancer (ALB) Health Checks
- Security Group Rules for ALB Targets
- Security Group Association
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
Application Load Balancer (ALB) Health Checks
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.
Visual reference
What to study next
Got this wrong? Here's your next step.
Review application Load Balancer (ALB) Health Checks, then practise related ANS-C01 questions on the same topic to reinforce the concept.
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FAQ
Questions learners often ask
What does this ANS-C01 question test?
Network Design — This question tests Network Design — Application Load Balancer (ALB) Health Checks.
What is the correct answer to this question?
The correct answer is: The ALB cannot route traffic to a private subnet — The most likely cause is that the Application Load Balancer cannot route traffic to an instance in a private subnet. By default, an ALB can only target instances in public subnets that have a route to an internet gateway. The private subnet lacks such a route, so the ALB cannot reach the EC2 instance for health checks, causing them to fail.
What should I do if I get this ANS-C01 question wrong?
Review application Load Balancer (ALB) Health Checks, then practise related ANS-C01 questions on the same topic to reinforce the concept.
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?
Application Load Balancer (ALB) Health Checks
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Last reviewed: Jun 24, 2026
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