The answer is that the web server on the EC2 instance is returning an HTTP 502 response. This is the most likely cause because an ALB health check 502 error directly indicates that the target application itself is failing, not that there is a network or configuration issue—the load balancer successfully reached the instance and received a 502 status code from the web server. On the AWS Certified SysOps Administrator Associate SOA-C02 exam, this question tests your ability to distinguish between application-level errors and infrastructure-level problems; a common trap is confusing a 502 with a connection timeout (which would be a 504) or assuming the health check port is wrong. Remember, a 502 means the application is alive but broken—think of it as the server saying “I got your request, but I can’t process it.” For your memory tip: 502 = Bad Gateway = Bad App Code.
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.
Network Topology
Refer to the exhibit. An EC2 instance is registered with an ALB target group. The health check returns 502. 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 web server on the instance is returning an HTTP 502 response.
Option C is correct because a 502 error from the target indicates the application on the instance is failing or not responding correctly to the health check. Option A is wrong because a 502 is not a connection timeout. Option B is wrong because the health check is on port 80, not 443. Option D is wrong because the instance is in the target group (but unhealthy).
Key principle: NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
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 instance is not reachable from the ALB due to a network ACL.
Why it's wrong here
Incorrect because that would cause connection timeout, not 502.
✗
The instance is not registered with the target group.
Why it's wrong here
Incorrect because the output shows the instance is registered but unhealthy.
✓
The web server on the instance is returning an HTTP 502 response.
Why this is correct
Correct because the health check receives a 502 from the target application.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Static NAT maps one inside address to one outside address.
✗
The health check path is configured for HTTPS but the instance only supports HTTP.
Why it's wrong here
Incorrect because the health check port is 80, not 443.
Common exam traps
Common exam trap: NAT rules depend on direction and matching traffic
NAT is not only about the public address. The inside/outside interface roles and the ACL or rule that matches traffic are just as important.
Trap categories for this question
Command / output trap
Incorrect because the output shows the instance is registered but unhealthy.
Detailed technical explanation
How to think about this question
NAT questions usually test address translation, overload/PAT behaviour, static mappings and whether the right traffic is being translated. Read the interface direction and address terms carefully.
KKey Concepts to Remember
Static NAT maps one inside address to one outside address.
PAT allows many inside hosts to share one public address using ports.
Inside local and inside global describe the private and translated addresses.
NAT ACLs identify traffic for translation, not always security filtering.
TExam Day Tips
→Identify inside and outside interfaces first.
→Check whether the scenario needs static NAT, dynamic NAT or PAT.
→Do not confuse NAT matching ACLs with normal packet-filtering intent.
Key takeaway
NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
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.
What to study next
Got this wrong? Here's your next step.
Review the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related SOA-C02 NAT questions on configuration and troubleshooting.
Networking and Content Delivery — This question tests Networking and Content Delivery — Static NAT maps one inside address to one outside address..
What is the correct answer to this question?
The correct answer is: The web server on the instance is returning an HTTP 502 response. — Option C is correct because a 502 error from the target indicates the application on the instance is failing or not responding correctly to the health check. Option A is wrong because a 502 is not a connection timeout. Option B is wrong because the health check is on port 80, not 443. Option D is wrong because the instance is in the target group (but unhealthy).
What should I do if I get this SOA-C02 question wrong?
Review the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related SOA-C02 NAT questions on configuration and troubleshooting.
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?
Static NAT maps one inside address to one outside address.
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