- A
The security group for the ALB does not allow inbound traffic from the internet.
Why wrong: This would cause complete failure, not intermittent timeouts. The ALB security group must allow inbound from internet, but missing this rule blocks all traffic.
- B
The ALB is not associated with an internet gateway.
Why wrong: The ALB does not need an internet gateway association; it uses an internet-facing scheme that routes through the VPC's internet gateway via route tables.
- C
The network ACL for the private subnets is blocking inbound traffic from the ALB subnets.
Correct. Network ACLs are stateless and must allow both inbound and outbound ephemeral port traffic. A missing inbound rule for ephemeral ports from ALB subnets will cause intermittent timeouts as some connections succeed and others fail.
- D
Cross-zone load balancing is disabled on the ALB.
Why wrong: Cross-zone load balancing being disabled would not cause timeouts; it would only cause uneven traffic distribution.
Why Users Experience Timeouts with ALB
This ANS-C01 practice question tests your understanding of network design. Match the stated requirement to the specific cloud service, access model, or configuration option — many options are valid in isolation but not for this scenario. A key principle to apply: ephemeral ports. 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 public and private subnets in two Availability Zones. An Application Load Balancer in the public subnets distributes traffic to EC2 instances in the private subnets. The security group for the EC2 instances allows inbound traffic from the ALB security group. Users report intermittent timeouts. 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 network ACL for the private subnets is blocking inbound traffic from the ALB subnets.
Intermittent timeouts are often caused by network ACL rules that block ephemeral ports used for return traffic. The ALB initiates connections to EC2 instances on random ephemeral ports (1024-65535). If the private subnet's network ACL denies inbound traffic on these ports from the ALB subnets, connections will fail intermittently. In contrast, a missing inbound rule on the ALB security group would result in consistent failures, not intermittent ones.
Key principle: Ephemeral ports
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 security group for the ALB does not allow inbound traffic from the internet.
Why it's wrong here
This would cause complete failure, not intermittent timeouts. The ALB security group must allow inbound from internet, but missing this rule blocks all traffic.
- ✗
The ALB is not associated with an internet gateway.
Why it's wrong here
The ALB does not need an internet gateway association; it uses an internet-facing scheme that routes through the VPC's internet gateway via route tables.
- ✓
The network ACL for the private subnets is blocking inbound traffic from the ALB subnets.
Why this is correct
Correct. Network ACLs are stateless and must allow both inbound and outbound ephemeral port traffic. A missing inbound rule for ephemeral ports from ALB subnets will cause intermittent timeouts as some connections succeed and others fail.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Ephemeral ports
- ✗
Cross-zone load balancing is disabled on the ALB.
Why it's wrong here
Cross-zone load balancing being disabled would not cause timeouts; it would only cause uneven traffic distribution.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Candidates often overlook that network ACLs are stateless and must allow ephemeral ports for return traffic. A missing rule for high ports (1024-65535) causes intermittent failures, not a complete outage.
Detailed technical explanation
How to think about this question
Under the hood, the ALB's security group acts as a stateful firewall at the instance level. For the ALB to accept incoming connections from clients, it must have an inbound rule allowing traffic from 0.0.0.0/0 on the listener port. Without this rule, the ALB will silently drop SYN packets, leading to connection timeouts from the client side. This is a common misconfiguration because the ALB's security group is often confused with the target group's security group, which only needs to allow traffic from the ALB's security group.
KKey Concepts to Remember
- Ephemeral ports
- Network ACL (NACL)
- Security group statefulness
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
Ephemeral ports
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.
Visual reference
What to study next
Got this wrong? Here's your next step.
Review ephemeral ports, 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 — Ephemeral ports.
What is the correct answer to this question?
The correct answer is: The network ACL for the private subnets is blocking inbound traffic from the ALB subnets. — Intermittent timeouts are often caused by network ACL rules that block ephemeral ports used for return traffic. The ALB initiates connections to EC2 instances on random ephemeral ports (1024-65535). If the private subnet's network ACL denies inbound traffic on these ports from the ALB subnets, connections will fail intermittently. In contrast, a missing inbound rule on the ALB security group would result in consistent failures, not intermittent ones.
What should I do if I get this ANS-C01 question wrong?
Review ephemeral ports, 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?
Ephemeral ports
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Last reviewed: Jun 30, 2026
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