- A
The instance does not have a public IP address
Without a public IP, the instance is not reachable from the internet.
- B
The network ACL inbound rule is incorrectly configured
Why wrong: The network ACL allows inbound HTTP.
- C
The security group is not associated with the instance
Why wrong: The scenario states the security group allows inbound HTTP.
- D
The route table does not have a route to the Internet Gateway
Why wrong: The scenario states both subnets have a route to the Internet Gateway.
Question 322 of 1,705
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 block of 10.0.0.0/16. It has two subnets: subnet A (10.0.1.0/24) and subnet B (10.0.2.0/24). Both subnets have a route to an Internet Gateway. An EC2 instance in subnet A has a security group that allows inbound HTTP from 0.0.0.0/0. The instance's network ACL allows inbound HTTP from 0.0.0.0/0. Users report they cannot access the web server. 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 instance does not have a public IP address
The most likely cause is that the EC2 instance does not have a public IP address. Even though the security group and network ACL allow inbound HTTP from 0.0.0.0/0, and the subnet has a route to an Internet Gateway (IGW), the instance must have a public IPv4 address (or an Elastic IP) for the IGW to translate its private IP to a public IP and route traffic from the internet. Without a public IP, the IGW cannot forward inbound traffic to the instance, making it unreachable from the internet.
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 instance does not have a public IP address
Why this is correct
Without a public IP, the instance is not reachable from the internet.
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 network ACL inbound rule is incorrectly configured
- ✗
The security group is not associated with the instance
Why it's wrong here
The scenario states the security group allows inbound HTTP.
- ✗
The route table does not have a route to the Internet Gateway
Why it's wrong here
The scenario states both subnets have a route to the Internet Gateway.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often assume that allowing traffic in both the security group and network ACL is sufficient, overlooking the fundamental requirement of a public IP address for internet-facing communication via an Internet Gateway.
Trap categories for this question
Scenario analysis trap
The scenario states the security group allows inbound HTTP.
Detailed technical explanation
How to think about this question
For an EC2 instance in a public subnet to be reachable from the internet, it must have a public IPv4 address (assigned at launch or via an Elastic IP) because the Internet Gateway performs 1:1 NAT, mapping the instance's private IP to its public IP. Without this mapping, the IGW drops inbound traffic destined for the instance. Additionally, the instance's operating system firewall (e.g., iptables) must also permit HTTP traffic, but that is not mentioned as a factor here.
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 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
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FAQ
Questions learners often ask
What does this ANS-C01 question test?
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 instance does not have a public IP address — The most likely cause is that the EC2 instance does not have a public IP address. Even though the security group and network ACL allow inbound HTTP from 0.0.0.0/0, and the subnet has a route to an Internet Gateway (IGW), the instance must have a public IPv4 address (or an Elastic IP) for the IGW to translate its private IP to a public IP and route traffic from the internet. Without a public IP, the IGW cannot forward inbound traffic to the instance, making it unreachable from the internet.
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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