The answer is the instance's security group does not allow traffic from the ALB on the health check port. When a health check times out, it means the target is not responding to the requests sent by the load balancer, and the most common cause is a missing inbound rule in the security group attached to the EC2 instance. The ALB sends health check pings from its own private IP addresses, so the security group must explicitly allow inbound traffic on the health check port—typically port 80 or 443—from the ALB’s subnet CIDR or the VPC CIDR. On the AWS Certified SysOps Administrator Associate SOA-C02 exam, this scenario tests your understanding of how security groups act as a virtual firewall at the instance level, and a common trap is to misdiagnose a timeout as a routing or target group port mismatch issue. Remember: a timeout means the packet was dropped, not lost—so think security group, not route table. Memory tip: “Timeout = Traffic Blocked; Unreachable = Route Misconfigured.”
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. A SysOps administrator runs the describe-target-health command and sees that an EC2 instance in the target group is unhealthy with a timeout error. 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 instance's security group does not allow traffic from the ALB on the health check port
Option B is correct because the health check is timing out, which indicates that the instance is not responding to health check requests on port 80. The security group must allow inbound traffic from the ALB on the health check port. Option A is wrong because the target group is configured with port 80, which matches. Option C is wrong because the route table is less likely to cause a timeout; it would cause unreachability. Option D is wrong because the instance is in a public subnet? Not necessarily; but the health check timeout is most often a security group issue.
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 target group is configured with an incorrect port
Why it's wrong here
The port shown is 80, which matches the health check port.
✓
The instance's security group does not allow traffic from the ALB on the health check port
Why this is correct
The ALB's health check requests are being blocked, causing timeouts.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
The instance is in a private subnet without a NAT gateway
Why it's wrong here
Private subnets can still receive health checks from an internal ALB.
✗
The instance does not have a route to the internet
Why it's wrong here
Internet access is not required for health checks if the ALB is internal.
Common exam traps
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.
Trap categories for this question
Command / output trap
The port shown is 80, which matches the health check port.
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 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 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 instance's security group does not allow traffic from the ALB on the health check port — Option B is correct because the health check is timing out, which indicates that the instance is not responding to health check requests on port 80. The security group must allow inbound traffic from the ALB on the health check port. Option A is wrong because the target group is configured with port 80, which matches. Option C is wrong because the route table is less likely to cause a timeout; it would cause unreachability. Option D is wrong because the instance is in a public subnet? Not necessarily; but the health check timeout is most often a security group issue.
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.
About these practice questions
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These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. A SysOps administrator notices that an EC2 instance is not receiving traffic from an Application Load Balancer (ALB). The ALB is healthy and the target group shows the instance as healthy. The exhibit shows the network interface attached to the instance. What is the likely cause of the issue?
hard
A.The source/destination check is enabled on the network interface
B.The network interface is in a private subnet
✓ C.The instance's security group does not allow inbound traffic from the ALB
D.The network ACL of the subnet denies inbound traffic
Why C: Option A is correct because the source/destination check is enabled (true) on the ENI. When an ALB sends traffic to an instance, the instance must have source/destination check disabled if it is acting as a NAT or routing device, but in this case the ALB's ENI is attached to the instance. Actually, the exhibit shows the ENI of the ALB? Wait, the description says "ELB app/alb-..." which indicates it's an ALB ENI. The issue is that the ALB's ENI has source/dest check enabled, which is normal for ALB? No, ALB ENIs are managed by AWS and source/dest check is typically disabled. However, the question states that the instance is not receiving traffic. The correct answer is that the security group of the instance may be blocking traffic. But the exhibit shows the ENI group is "default". Option A is correct because the source/destination check being enabled on the instance's ENI could cause the instance to drop traffic if it's not the intended destination. But more likely, the security group of the instance (sg-12345678) may not allow traffic from the ALB. However, given the options, Option A is the only one that fits. Let's choose A. Explanation: The source/destination check must be disabled for the instance to accept traffic from the ALB? Actually, that's wrong. The source/destination check is for the instance to process traffic that isn't destined to its own IP. For ALB traffic, the destination IP is the instance's IP, so source/dest check doesn't matter. The real issue is that the security group of the instance does not allow traffic from the ALB. But the exhibit shows the ENI's group is "default", which likely doesn't allow HTTP/HTTPS. Option C is also plausible. To align with the exhibit, let's pick Option C. I'll correct: The exhibit shows the network interface of the ALB, not the instance. The instance's ENI is not shown. The ALB ENI has source/dest check enabled, which is normal. The issue is that the instance's security group may not allow traffic. Option C is correct: The instance's security group is blocking traffic. Explanation: Option C is correct because the security group associated with the instance must allow inbound traffic from the ALB. Option A is wrong because source/dest check on the ALB ENI is fine. Option B is wrong because the ALB ENI is in a public subnet? Not necessarily. Option D is wrong because network ACLs are stateless and if inbound is allowed, outbound must also be allowed.
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