- A
The security group of the EC2 instance blocks outbound traffic.
Why wrong: Security groups are stateful; outbound is allowed by default.
- B
The route table in the public subnet does not have a default route to the internet gateway.
Why wrong: Public subnet route table needs default route to IGW for NAT gateway to work.
- C
The network ACL of the private subnet blocks inbound traffic from the NAT gateway.
Why wrong: NACLs are stateless; but the problem is outbound.
- D
The NAT gateway's Elastic IP is not allowed by the destination's firewall.
Destination may block the public IP.
Question 1,101 of 1,705
NAT Gateway Elastic IP Blocked by Destination Firewall
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 has a VPC with a public subnet and a private subnet. An EC2 instance in the private subnet needs to initiate outbound connections to the internet. The company has a NAT gateway in the public subnet. The NAT gateway has an Elastic IP. The private subnet route table has a default route pointing to the NAT gateway. However, the EC2 instance cannot reach the internet. 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 NAT gateway's Elastic IP is not allowed by the destination's firewall.
Option D is correct because the NAT gateway's Elastic IP is a public IP address that the destination server sees as the source of the outbound traffic. If the destination's firewall (e.g., a security appliance or an internet-based service) explicitly blocks traffic from that Elastic IP, the EC2 instance will be unable to reach the internet, even though the VPC routing and NAT configuration are correct. This is a common scenario when the destination uses IP-based allowlists or blacklists.
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 security group of the EC2 instance blocks outbound traffic.
Why it's wrong here
Security groups are stateful; outbound is allowed by default.
- ✗
The route table in the public subnet does not have a default route to the internet gateway.
Why it's wrong here
Public subnet route table needs default route to IGW for NAT gateway to work.
- ✗
The network ACL of the private subnet blocks inbound traffic from the NAT gateway.
Why it's wrong here
NACLs are stateless; but the problem is outbound.
- ✓
The NAT gateway's Elastic IP is not allowed by the destination's firewall.
Why this is correct
Destination may block the public IP.
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.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The ANS-C01 exam often tests the misconception that NAT gateway or internet gateway configuration is the sole cause of connectivity failures, while in reality, external firewalls or destination-side IP filtering can independently block traffic even when all AWS-side networking is correctly configured.
Detailed technical explanation
How to think about this question
NAT gateways perform source network address translation (SNAT), replacing the private IP of the EC2 instance with the NAT gateway's Elastic IP for outbound traffic. The destination server sees only the Elastic IP, so any firewall rules (e.g., IPtables, AWS Network Firewall, or third-party security appliances) that filter based on source IP will directly impact connectivity. Additionally, NAT gateways do not support port forwarding or allow inbound connections initiated from the internet, which is a key distinction from NAT instances.
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
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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: The NAT gateway's Elastic IP is not allowed by the destination's firewall. — Option D is correct because the NAT gateway's Elastic IP is a public IP address that the destination server sees as the source of the outbound traffic. If the destination's firewall (e.g., a security appliance or an internet-based service) explicitly blocks traffic from that Elastic IP, the EC2 instance will be unable to reach the internet, even though the VPC routing and NAT configuration are correct. This is a common scenario when the destination uses IP-based allowlists or blacklists.
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
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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