The answer is that the network ACLs are blocking the intra-VPC traffic. This is because network ACLs are stateless, meaning they require explicit inbound and outbound rules for each direction of traffic, whereas security groups are stateful and automatically allow return traffic. Even if the security groups on both EC2 instances permit all traffic from each other’s private IP, the default custom NACL for each subnet denies all inbound and outbound traffic, effectively blocking communication between the two instances in different subnets. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this scenario tests your understanding of the fundamental difference between stateful security groups and stateless NACLs, a common trap where candidates assume security group rules alone guarantee connectivity. A key memory tip: think of NACLs as a “stateless bouncer” that checks every packet entering and leaving the subnet, while security groups are a “stateful friend” who remembers who you already talked to.
ANS-C01 Network Design Practice Question
This ANS-C01 practice question tests your understanding of network design. 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 has a VPC with a CIDR of 10.0.0.0/16. They have two subnets: subnet A (10.0.1.0/24) and subnet B (10.0.2.0/24). They launch an EC2 instance in subnet A and another in subnet B. The security groups for both instances allow all traffic from the other instance's private IP. However, the instances cannot communicate. 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 network ACLs are blocking the traffic
The most likely cause is that the network ACLs (NACLs) are blocking the traffic. Security groups are stateful and allow return traffic automatically, but NACLs are stateless and require explicit inbound and outbound rules for both directions. By default, custom NACLs deny all traffic, so even though the security groups permit communication, the NACLs on subnets A and B must allow the traffic. The correct answer is B.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
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 instances are in different Availability Zones
Why it's wrong here
Instances in same VPC can communicate across AZs.
✓
The network ACLs are blocking the traffic
Why this is correct
NACLs are stateless and need to allow both outbound and inbound traffic for each direction.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
The security groups are stateful and block return traffic
Why it's wrong here
Security groups are stateful, automatically allow return traffic.
✗
The route tables do not have a local route
Why it's wrong here
VPC automatically adds local route.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often assume security groups are the only firewall layer or forget that NACLs are stateless and require explicit rules for both directions, leading them to overlook NACL misconfigurations when security groups appear permissive.
Detailed technical explanation
How to think about this question
Network ACLs operate at the subnet level and are stateless, meaning each packet is evaluated independently against inbound and outbound rules. For two instances in different subnets to communicate, both the inbound rule on the destination subnet's NACL and the outbound rule on the source subnet's NACL must allow the traffic. By default, custom NACLs deny all traffic, while the default NACL allows all traffic. This contrasts with security groups, which are stateful and automatically track connection state.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
TExam Day Tips
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
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.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Network Design — This question tests Network Design — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The network ACLs are blocking the traffic — The most likely cause is that the network ACLs (NACLs) are blocking the traffic. Security groups are stateful and allow return traffic automatically, but NACLs are stateless and require explicit inbound and outbound rules for both directions. By default, custom NACLs deny all traffic, so even though the security groups permit communication, the NACLs on subnets A and B must allow the traffic. The correct answer is B.
What should I do if I get this ANS-C01 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
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?
Read the scenario before looking for a memorised answer.
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 →
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.
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.