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. 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 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 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 ALB does not allow inbound traffic from the internet.
The most likely cause is that the ALB's security group does not allow inbound traffic from the internet (0.0.0.0/0) on the listener port (e.g., 80 or 443). Without this rule, the ALB will reject incoming client requests, causing intermittent timeouts as users attempt to connect. The ALB must have an inbound rule permitting traffic from the internet to receive and forward requests to the targets.
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 security group for the ALB does not allow inbound traffic from the internet.
Why this is correct
ALB security group must allow inbound from clients.
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.
✗
The ALB is not associated with an internet gateway.
Why it's wrong here
ALB can route traffic without an IGW if it has public IPs.
✗
The network ACL for the private subnets is blocking inbound traffic from the ALB subnets.
Why it's wrong here
NACLs must allow ephemeral ports; this is a common misconfiguration but less likely than security group.
✗
Cross-zone load balancing is disabled on the ALB.
Why it's wrong here
Cross-zone LB is enabled by default.
Common exam traps
Common exam trap: answer the scenario, not the keyword
AWS often tests the misconception that the ALB's security group only needs to allow outbound traffic to targets, but in reality, the ALB's security group must also allow inbound traffic from clients to receive requests.
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
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 ALB does not allow inbound traffic from the internet. — The most likely cause is that the ALB's security group does not allow inbound traffic from the internet (0.0.0.0/0) on the listener port (e.g., 80 or 443). Without this rule, the ALB will reject incoming client requests, causing intermittent timeouts as users attempt to connect. The ALB must have an inbound rule permitting traffic from the internet to receive and forward requests to the targets.
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.
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Question Discussion
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