- A
The source/destination check is enabled, which prevents the instance from sending traffic to the internet if it is not the source of the traffic.
Why wrong: The source/destination check is enabled by default on EC2 instances and is normal; it does not prevent the instance from reaching the internet. This is not the cause.
- B
The network interface's attachment status is 'attached', but the instance may not have a public IP address.
An instance in a public subnet needs a public IP address for the internet gateway to route traffic to and from the internet. Without it, even with proper routes and security groups, internet access fails. This is the most likely cause.
- C
The route table for the subnet does not have a 0.0.0.0/0 route pointing to the internet gateway.
Why wrong: The stem states the subnet has a route to the internet gateway, so the default route is present. This is not the issue.
- D
The security group is not associated with the network interface.
Why wrong: The security group is associated with the network interface and allows outbound traffic, so this is not the cause.
ANS-C01 Public IP address Practice Question
This ANS-C01 practice question tests your understanding of network management and operations. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. A key principle to apply: public IP address. 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.
An EC2 instance with the attached network interface shown above is unable to reach the internet. The instance is in a public subnet with a route to an internet gateway. The security group allows outbound traffic. 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
The network interface's attachment status is 'attached', but the instance may not have a public IP address.
The instance is in a public subnet with a route to an internet gateway, and the security group allows outbound traffic. However, for an instance in a public subnet to reach the internet directly, it must have a public IP address (either assigned automatically or as an Elastic IP). Without a public IP, the internet gateway cannot translate the instance's private IP to a public one, so outbound traffic fails. The route table is correct (contains 0.0.0.0/0 pointing to the internet gateway), source/destination check enabled is normal and does not block internet access, and the security group is associated and allows outbound traffic. Therefore, the most likely cause is the lack of a public IP address.
Key principle: Public IP address
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 source/destination check is enabled, which prevents the instance from sending traffic to the internet if it is not the source of the traffic.
Why it's wrong here
The source/destination check is enabled by default on EC2 instances and is normal; it does not prevent the instance from reaching the internet. This is not the cause.
- ✓
The network interface's attachment status is 'attached', but the instance may not have a public IP address.
Why this is correct
An instance in a public subnet needs a public IP address for the internet gateway to route traffic to and from the internet. Without it, even with proper routes and security groups, internet access fails. This is the most likely cause.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Public IP address
- ✗
The route table for the subnet does not have a 0.0.0.0/0 route pointing to the internet gateway.
Why it's wrong here
The stem states the subnet has a route to the internet gateway, so the default route is present. This is not the issue.
- ✗
The security group is not associated with the network interface.
Why it's wrong here
The security group is associated with the network interface and allows outbound traffic, so this is not the cause.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap is that candidates often overlook the requirement for a public IP address when the route table is correctly configured. They may mistakenly focus on route issues or source/destination check.
Detailed technical explanation
How to think about this question
Treat this as a scenario question. Identify the problem, the constraint, and the best action. Then compare each option against those facts.
KKey Concepts to Remember
- Public IP address
- Internet Gateway
- Route Table
- Source/Destination Check
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
Public IP address
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.
Visual reference
What to study next
Got this wrong? Here's your next step.
Review public IP address, then practise related ANS-C01 questions on the same topic to reinforce the concept.
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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 — Public IP address.
What is the correct answer to this question?
The correct answer is: The network interface's attachment status is 'attached', but the instance may not have a public IP address. — The instance is in a public subnet with a route to an internet gateway, and the security group allows outbound traffic. However, for an instance in a public subnet to reach the internet directly, it must have a public IP address (either assigned automatically or as an Elastic IP). Without a public IP, the internet gateway cannot translate the instance's private IP to a public one, so outbound traffic fails. The route table is correct (contains 0.0.0.0/0 pointing to the internet gateway), source/destination check enabled is normal and does not block internet access, and the security group is associated and allows outbound traffic. Therefore, the most likely cause is the lack of a public IP address.
What should I do if I get this ANS-C01 question wrong?
Review public IP address, then practise related ANS-C01 questions on the same topic to reinforce the concept.
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?
Public IP address
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Last reviewed: Jun 20, 2026
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