The answer is a misconfigured network ACL blocking the return traffic from health checks. The NACL is stateless, meaning it evaluates inbound and outbound traffic independently; while the outbound rule allows ephemeral ports for responses, the inbound rule must explicitly allow the ALB’s health check return traffic on ephemeral ports (1024-65535) from the ALB’s subnet range. Without this inbound rule, the health check responses from the EC2 instances are dropped, causing intermittent failures even though the security group correctly permits port 80 from the ALB. On the AWS Certified SysOps Administrator Associate SOA-C02 exam, this scenario tests your understanding of stateless vs. stateful filtering—a common trap is assuming security group rules alone suffice. Remember: NACLs require explicit inbound ephemeral port rules for any return traffic initiated from inside the subnet. Memory tip: “Stateless needs both doors open—inbound ephemeral for outbound replies.”
SOA-C02 Networking and Content Delivery Practice Question
This SOA-C02 practice question tests your understanding of networking and content delivery. 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 runs a critical application on EC2 instances behind an Application Load Balancer (ALB) across three Availability Zones. Each AZ has one public and one private subnet. The EC2 instances are in the private subnets. The ALB is internet-facing. Recently, during a traffic spike, some users experienced intermittent timeouts. The SysOps administrator reviews the ALB access logs and finds that the timeouts correspond to periods when the target group had 'unhealthy' instances. The health check is configured to check a health endpoint on port 80 with a path of '/health'. The SysOps administrator checks the EC2 instances and finds that the health endpoint responds correctly. However, the health checks are failing intermittently. The administrator notices that the security group for the EC2 instances allows inbound traffic from the ALB's security group on port 80. The network ACL for the private subnets allows inbound HTTP and outbound ephemeral ports. What is the MOST likely cause of the health check failures?
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 network ACL for the private subnets is blocking the return traffic from health checks.
Option B is correct. The NACL is stateless and must allow inbound ephemeral ports for the return traffic from health checks. The current NACL allows inbound HTTP (port 80) but does not explicitly allow inbound ephemeral ports (1024-65535) from the ALB's IP range. This causes the health check responses to be dropped. Option A is wrong because the security group is configured correctly. Option C is wrong because the health endpoint works when tested locally. Option D is wrong because cross-zone load balancing does not affect health checks.
Key principle: Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.
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 health check endpoint on the EC2 instances is not responding correctly.
Why it's wrong here
The endpoint works when tested locally.
✗
The security group on the EC2 instances is blocking health check traffic from the ALB.
Why it's wrong here
The security group allows HTTP from the ALB's security group, so it is not blocking.
✗
The ALB does not have cross-zone load balancing enabled.
Why it's wrong here
Cross-zone load balancing does not affect health checks.
✓
The network ACL for the private subnets is blocking the return traffic from health checks.
Why this is correct
NACL must allow inbound ephemeral ports for health check responses.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Common exam trap: usable hosts are not the same as total addresses
Subnetting questions often tempt you into counting all addresses. In normal IPv4 subnets, the network and broadcast addresses are not usable host addresses.
Detailed technical explanation
How to think about this question
Subnetting questions test whether you can identify the network, broadcast address, usable range, mask and correct subnet. Slow down enough to calculate the block size correctly.
KKey Concepts to Remember
CIDR notation defines the prefix length.
Block size helps identify subnet boundaries.
Network and broadcast addresses are not usable hosts in normal IPv4 subnets.
The required host count determines the smallest suitable subnet.
TExam Day Tips
→Write the block size before choosing the subnet.
→Check whether the question asks for hosts, subnets or a specific address range.
→Do not confuse /24, /25, /26 and /27 host counts.
Key takeaway
Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.
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.
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related SOA-C02 subnetting questions on CIDR, address ranges, and subnet selection.
Networking and Content Delivery — This question tests Networking and Content Delivery — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: The network ACL for the private subnets is blocking the return traffic from health checks. — Option B is correct. The NACL is stateless and must allow inbound ephemeral ports for the return traffic from health checks. The current NACL allows inbound HTTP (port 80) but does not explicitly allow inbound ephemeral ports (1024-65535) from the ALB's IP range. This causes the health check responses to be dropped. Option A is wrong because the security group is configured correctly. Option C is wrong because the health endpoint works when tested locally. Option D is wrong because cross-zone load balancing does not affect health checks.
What should I do if I get this SOA-C02 question wrong?
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related SOA-C02 subnetting questions on CIDR, address ranges, and subnet selection.
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?
CIDR notation defines the prefix length.
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