The answer is a misconfigured security group on the EC2 instances that does not allow inbound traffic from the ALB’s security group. This is because ALB health checks originate from the ALB’s own subnet IPs, not from the NAT Gateway, so the instances must explicitly permit inbound traffic from the ALB’s security group on the health check port; without this rule, the ALB sees the targets as unhealthy, causing intermittent timeouts. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this scenario tests your understanding of how security group traffic flows differ from route table or NAT Gateway logic—a common trap is assuming health checks need outbound internet access. Remember the memory tip: “Health checks are inbound, not outbound—allow the ALB’s SG, not the NAT.”
ANS-C01 Network Implementation Practice Question
This ANS-C01 practice question tests your understanding of network implementation. 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 deployed an Application Load Balancer (ALB) in front of a fleet of EC2 instances. Users report intermittent timeouts. The ALB's target group health checks are failing for some instances. The instances are in private subnets with a single NAT Gateway. 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 on the EC2 instances does not allow inbound traffic from the ALB's security group.
Health checks from the ALB originate from the ALB's subnet and do not require a NATGateway. However, if the security group for the instances does not allow inbound traffic from the ALB's security group, health checks will fail. Option A (NAT Gateway) is unrelated; Option B (route table) would affect outbound traffic; Option C (instance type) is unlikely.
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 security group on the EC2 instances does not allow inbound traffic from the ALB's security group.
Why this is correct
ALB health checks originate from the ALB's security group; if not allowed, health checks fail.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
The route tables in the private subnets do not have a default route pointing to the NAT Gateway.
Why it's wrong here
Missing default route affects outbound internet traffic, not health checks from ALB.
✗
The EC2 instances are of an unsupported instance type for ALB target groups.
Why it's wrong here
ALB supports all current EC2 instance types.
✗
The NAT Gateway is in a single Availability Zone, causing traffic to fail during an AZ outage.
Why it's wrong here
Health checks do not traverse the NATGateway; they are sent directly from the ALB.
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.
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
An e-commerce site experiences heavy traffic on Black Friday and near-zero traffic during off-peak weeks. Rather than provisioning permanent large VMs, the team uses auto-scaling groups that add capacity automatically under load and reduce it overnight. Questions like this test whether you understand elasticity, availability zones, and cloud compute scaling patterns.
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 ANS-C01 subnetting questions on CIDR, address ranges, and subnet selection.
Network Implementation — This question tests Network Implementation — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: The security group on the EC2 instances does not allow inbound traffic from the ALB's security group. — Health checks from the ALB originate from the ALB's subnet and do not require a NAT Gateway. However, if the security group for the instances does not allow inbound traffic from the ALB's security group, health checks will fail. Option A (NAT Gateway) is unrelated; Option B (route table) would affect outbound traffic; Option C (instance type) is unlikely.
What should I do if I get this ANS-C01 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 ANS-C01 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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