- A
VPC Flow Logs misconfiguration.
Why wrong: Flow logs are passive.
- B
Internet gateway not attached to the VPC.
Why wrong: Internal traffic does not use IGW.
- C
Security group or network ACL rules blocking port 443.
Port 443 is likely blocked by a firewall rule.
- D
Missing route in the route table.
Why wrong: Routes are not port-specific; ICMP works.
Question 992 of 1,705
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 network engineer is troubleshooting connectivity between two EC2 instances in the same VPC but different subnets. The instances can ping each other's private IP addresses, but traffic on TCP port 443 fails. 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 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
✓
Security group or network ACL rules blocking port 443.
Since the instances can ping each other, ICMP traffic is working, which rules out routing issues and confirms that the instances are reachable at Layer 3. TCP port 443 failing specifically indicates that a firewall rule is blocking the HTTPS traffic. Security groups are stateful and operate at the instance level, while network ACLs are stateless and operate at the subnet level; either can block port 443 while allowing ICMP, making this the most likely cause.
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.
- ✗
VPC Flow Logs misconfiguration.
Why it's wrong here
Flow logs are passive.
- ✗
Internet gateway not attached to the VPC.
Why it's wrong here
Internal traffic does not use IGW.
- ✓
Security group or network ACL rules blocking port 443.
Why this is correct
Port 443 is likely blocked by a firewall rule.
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.
- ✗
Missing route in the route table.
Why it's wrong here
Routes are not port-specific; ICMP works.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The ANS-C01 exam often tests the distinction between stateful security groups and stateless network ACLs, and the trap here is that candidates assume ping success means all Layer 4 traffic is fine, ignoring that security groups and ACLs can selectively block specific ports while allowing others.
Detailed technical explanation
How to think about this question
Security groups are stateful, meaning that if you allow inbound traffic on port 443, the return traffic is automatically allowed regardless of outbound rules. Network ACLs are stateless and require explicit rules for both inbound and outbound traffic; a common misconfiguration is allowing inbound HTTPS but forgetting the ephemeral port range (1024-65535) for outbound responses. In real-world scenarios, engineers often test with ping (ICMP) and assume all traffic works, but ICMP is frequently allowed by default in security groups and ACLs while TCP ports are explicitly denied.
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
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.
Visual reference
What to study next
Got this wrong? Here's your next step.
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FAQ
Questions learners often ask
What does this ANS-C01 question test?
Network Implementation — This question tests Network Implementation — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Security group or network ACL rules blocking port 443. — Since the instances can ping each other, ICMP traffic is working, which rules out routing issues and confirms that the instances are reachable at Layer 3. TCP port 443 failing specifically indicates that a firewall rule is blocking the HTTPS traffic. Security groups are stateful and operate at the instance level, while network ACLs are stateless and operate at the subnet level; either can block port 443 while allowing ICMP, making this the most likely cause.
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.
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Last reviewed: Jul 4, 2026
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