- A
The network ACL associated with the EC2 instances' subnet does not have an outbound rule to allow traffic from the EC2 instances to the ALB on ephemeral ports.
The network ACL is stateless and must allow return traffic. Missing outbound rules cause REJECT.
- B
The ALB's security group is blocking inbound traffic from the EC2 instances on the response path.
Why wrong: The ALB does not have a security group that affects traffic to targets; target security group handles inbound.
- C
The ALB's target group health check is misconfigured, causing the ALB to mark instances as unhealthy and stop sending traffic.
Why wrong: Health checks use the same security group rules; if instances are healthy, traffic should flow.
- D
The security group on the EC2 instances is stateful and automatically allows return traffic; the issue cannot be security group related.
Why wrong: Security groups are stateful, but network ACLs are not; the issue is likely with the ACL, not the security group.
Quick Answer
The answer is the missing outbound rule in the network ACL (NACL) for ephemeral ports. This is correct because NACLs are stateless, meaning they evaluate inbound and outbound traffic independently; when the ALB sends a SYN packet to an EC2 instance on port 8080, the instance’s response (SYN-ACK) uses a random ephemeral port as its source, and if the subnet’s NACL lacks an outbound rule allowing this return traffic, the packets are rejected, causing intermittent connectivity. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this scenario tests your understanding of stateless versus stateful filtering and how ephemeral port ranges (typically 1024-65535) must be explicitly allowed in both directions for NACLs. A common trap is assuming security group statefulness compensates for missing NACL rules, but security groups only apply at the instance level, not the subnet boundary. Memory tip: NACLs are “no exceptions” — they require explicit outbound ephemeral port rules for any inbound connection to succeed.
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 is deploying a multi-tier web application on AWS. The application consists of an Application Load Balancer (ALB), a fleet of EC2 instances in an Auto Scaling group across three Availability Zones, and an Amazon RDS for MySQL database. The ALB has a target group that routes traffic to the EC2 instances on TCP port 8080. The security group for the EC2 instances allows inbound traffic from the ALB's security group on port 8080. Users report intermittent connectivity issues to the application. A network engineer reviews the VPC Flow Logs and notices that traffic from the ALB to the EC2 instances is being recorded as 'REJECT' for some requests. What is the most likely cause of this issue?
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 choices
Why each option matters
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 associated with the EC2 instances' subnet does not have an outbound rule to allow traffic from the EC2 instances to the ALB on ephemeral ports.
Option A is correct because the network ACL is stateless and must allow both inbound and outbound traffic for ephemeral ports. If the outbound rule is missing, SYN-ACK packets from the EC2 instance to the ALB will be dropped, causing the ALB to see a timeout or reject. Option B is incorrect because target group health checks use the same security group rules; if health checks succeed, connectivity should work. Option C is incorrect because the ALB itself does not have a security group that affects traffic to targets; it uses the target group's security group. Option D is incorrect because network ACLs are stateless and require explicit rules for return traffic; the security group stateful behavior does not override ACL rules.
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 network ACL associated with the EC2 instances' subnet does not have an outbound rule to allow traffic from the EC2 instances to the ALB on ephemeral ports.
- ✗
The ALB's security group is blocking inbound traffic from the EC2 instances on the response path.
Why it's wrong here
The ALB does not have a security group that affects traffic to targets; target security group handles inbound.
- ✗
The ALB's target group health check is misconfigured, causing the ALB to mark instances as unhealthy and stop sending traffic.
Why it's wrong here
Health checks use the same security group rules; if instances are healthy, traffic should flow.
- ✗
The security group on the EC2 instances is stateful and automatically allows return traffic; the issue cannot be security group related.
Why it's wrong here
Security groups are stateful, but network ACLs are not; the issue is likely with the ACL, not the security group.
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
A media company stores terabytes of video archives that are accessed once a year for audit purposes. Moving these objects to a cold storage tier (Azure Archive, S3 Glacier, or Google Nearline) costs a fraction of hot storage. Questions like this test whether you understand storage tiers, access frequency tradeoffs, and retrieval latency requirements.
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 — study guide chapter
Learn the concepts, then practise the questions
- →
Network Implementation practice questions
Targeted practice on this topic area only
- →
All ANS-C01 questions
1,705 questions across all exam domains
- →
AWS Certified Advanced Networking Specialty ANS-C01 study guide
Full concept coverage aligned to exam objectives
- →
ANS-C01 practice test guide
How to use practice tests most effectively before exam day
Related practice questions
Related ANS-C01 practice-question pages
Use these pages to review the topic behind this question. This is how one missed question becomes focused revision.
Network Management and Operations practice questions
Practise ANS-C01 questions linked to Network Management and Operations.
Network Security, Compliance and Governance practice questions
Practise ANS-C01 questions linked to Network Security, Compliance and Governance.
Network Design practice questions
Practise ANS-C01 questions linked to Network Design.
Network Implementation practice questions
Practise ANS-C01 questions linked to Network Implementation.
ANS-C01 fundamentals practice questions
Practise ANS-C01 questions linked to ANS-C01 fundamentals.
ANS-C01 scenario practice questions
Practise ANS-C01 questions linked to ANS-C01 scenario.
ANS-C01 troubleshooting practice questions
Practise ANS-C01 questions linked to ANS-C01 troubleshooting.
Practice this exam
Start a free ANS-C01 practice session
Short sessions build daily habit. Longer sessions build exam-day stamina. Try a timed session to simulate real conditions.
FAQ
Questions learners often ask
What does this ANS-C01 question test?
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 network ACL associated with the EC2 instances' subnet does not have an outbound rule to allow traffic from the EC2 instances to the ALB on ephemeral ports. — Option A is correct because the network ACL is stateless and must allow both inbound and outbound traffic for ephemeral ports. If the outbound rule is missing, SYN-ACK packets from the EC2 instance to the ALB will be dropped, causing the ALB to see a timeout or reject. Option B is incorrect because target group health checks use the same security group rules; if health checks succeed, connectivity should work. Option C is incorrect because the ALB itself does not have a security group that affects traffic to targets; it uses the target group's security group. Option D is incorrect because network ACLs are stateless and require explicit rules for return traffic; the security group stateful behavior does not override ACL rules.
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.
About these practice questions
Courseiva creates original exam-style practice questions with explanations and wrong-answer analysis. It does not publish real exam questions, exam dumps, or protected exam content. Learn why practice questions differ from exam dumps →
Last reviewed: Jun 20, 2026
This ANS-C01 practice question is part of Courseiva's free Amazon Web Services certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the ANS-C01 exam.
Question Discussion
Share a tip, memory trick, or ask about the reasoning behind this question. Do not post real exam questions, leaked content, braindumps, or copyrighted exam material. Comments are moderated and may be removed without notice.
Sign in to join the discussion.