- A
The network ACL inbound rule is blocking ephemeral ports.
Why wrong: Network ACLs are stateless, but the default NACL allows all traffic, so this is unlikely.
- B
The instances are behind a proxy that is not configured.
Why wrong: No proxy is mentioned, and SSH does not require a proxy.
- C
The security group outbound rules are not allowing return traffic.
Security groups are stateful, but if outbound rules are restrictive, return traffic may be blocked. However, by default, security groups allow all outbound. The actual cause is likely the NAT gateway's public IP changing or a missing route.
- D
The NAT gateway does not have an Elastic IP associated.
Why wrong: A NAT gateway without an Elastic IP cannot communicate with the internet, but if it had one, it would work. Intermittency suggests a different issue.
Quick Answer
The answer is the security group outbound rules not allowing return traffic. This is correct because security groups are stateful, meaning they automatically allow return traffic for established connections only if the outbound rules permit the corresponding ephemeral ports—typically in the range 1024-65535 for SSH responses. If outbound rules are too restrictive, the NAT gateway can forward the initial SSH request, but the return packets from the EC2 instance are dropped, causing intermittent failures that appear random. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this scenario tests your understanding of stateful versus stateless filtering and how session tracking interacts with NAT gateways. A common trap is blaming network ACLs, but since they are stateless and set to allow all, they are not the culprit. Remember the mnemonic: “Stateful needs return flow—check outbound rules, not just inbound.”
ANS-C01 Network Management and Operations Practice Question
This ANS-C01 practice question tests your understanding of network management and operations. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. 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 experiencing intermittent SSH connection failures to their EC2 instances in a VPC. The instances are in a private subnet with a NAT gateway. The security group allows inbound SSH from the corporate CIDR. The network ACL is set to default allow all. The route table has a route to the NAT gateway for 0.0.0.0/0. What is the most likely cause of the intermittent failures?
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 security group outbound rules are not allowing return traffic.
Option C is correct because security groups are stateful, but if the security group does not allow inbound ICMP or the specific ephemeral ports for SSH, connections may fail intermittently due to session tracking issues. However, the more common cause in such scenarios is the NAT gateway's IP address changing if it is not associated with an Elastic IP, or the security group outbound rule not allowing return traffic. Option A is plausible but less likely as NACLs are stateless and would not cause intermittent issues. Option B is incorrect because the NAT gateway does not use an Elastic IP if not configured. Option D is incorrect because a proxy is not typically needed.
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 inbound rule is blocking ephemeral ports.
Why it's wrong here
Network ACLs are stateless, but the default NACL allows all traffic, so this is unlikely.
- ✗
The instances are behind a proxy that is not configured.
Why it's wrong here
No proxy is mentioned, and SSH does not require a proxy.
- ✓
The security group outbound rules are not allowing return traffic.
Why this is correct
Security groups are stateful, but if outbound rules are restrictive, return traffic may be blocked. However, by default, security groups allow all outbound. The actual cause is likely the NAT gateway's public IP changing or a missing route.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
CIDR notation defines the prefix length.
- ✗
The NAT gateway does not have an Elastic IP associated.
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 company's IT admin needs to give a contractor read-only access to production logs without sharing account credentials. Using role-based access control (RBAC) and temporary scoped permissions — not a permanent shared password — is the correct pattern. Questions like this test whether you can apply least-privilege access across cloud identity services.
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.
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Network Management and Operations — study guide chapter
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FAQ
Questions learners often ask
What does this ANS-C01 question test?
Network Management and Operations — This question tests Network Management and Operations — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: The security group outbound rules are not allowing return traffic. — Option C is correct because security groups are stateful, but if the security group does not allow inbound ICMP or the specific ephemeral ports for SSH, connections may fail intermittently due to session tracking issues. However, the more common cause in such scenarios is the NAT gateway's IP address changing if it is not associated with an Elastic IP, or the security group outbound rule not allowing return traffic. Option A is plausible but less likely as NACLs are stateless and would not cause intermittent issues. Option B is incorrect because the NAT gateway does not use an Elastic IP if not configured. Option D is incorrect because a proxy is not typically needed.
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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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.
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