The answer is that the subnet does not have auto-assign public IP enabled, specifically because the CloudFormation template omits the 'MapPublicIpOnLaunch' property set to 'true'. Without this setting, EC2 instances launched in the subnet lack a public IPv4 address, which is essential for the Internet Gateway (IGW) to perform source NAT for outbound traffic; even with a default route (0.0.0.0/0) pointing to the IGW, the instance remains unreachable from the internet. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this scenario tests your understanding of how CloudFormation subnet auto-assign public IP interacts with IGW routing—a common trap is assuming a route table alone guarantees internet access. Remember that a public subnet requires both a route to the IGW and the auto-assign public IP attribute enabled at the subnet level. A useful memory tip: "No public IP, no NAT—no internet."
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.
Refer to the exhibit. A developer created this CloudFormation template to create a public subnet. However, instances in the subnet cannot access 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 the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The subnet does not have auto-assign public IP enabled
Option A is correct because the CloudFormation template creates a public subnet but does not set the 'MapPublicIpOnLaunch' property to 'true'. Without this setting, EC2 instances launched in the subnet do not automatically receive a public IPv4 address, which is required for internet-bound traffic to be routed through the Internet Gateway (IGW). Even if the route table has a default route (0.0.0.0/0) pointing to the IGW, the instance lacks a public IP, so the IGW cannot perform source NAT for outbound traffic, making internet access impossible.
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 subnet does not have auto-assign public IP enabled
Why this is correct
Instances need public IPs for internet access; template does not enable it.
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 route to the internet gateway is missing
Why it's wrong here
The template has a route to 0.0.0.0/0 via IGW.
✗
The route table is not associated with the subnet
Why it's wrong here
The template has a SubnetRouteTableAssociation.
✗
The internet gateway is not attached to the VPC
Why it's wrong here
The template has VPCGatewayAttachment.
Common exam traps
Common exam trap: answer the scenario, not the keyword
AWS often tests the misconception that a default route to an IGW alone guarantees internet access, but the trap here is that the instance must have a public IP address for the IGW to perform source NAT, and auto-assign public IP is disabled by default in CloudFormation unless explicitly enabled.
Detailed technical explanation
How to think about this question
Under the hood, the Internet Gateway performs one-to-one NAT for instances with public IPv4 addresses. When an instance sends traffic to the internet, the IGW translates the source private IP to the instance's public IP. Without a public IP assigned at launch (via 'MapPublicIpOnLaunch' or an Elastic IP), the IGW has no public address to use for translation, so packets are dropped. This behavior is defined in the AWS VPC documentation and is independent of route table configurations.
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.
What to study next
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
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 subnet does not have auto-assign public IP enabled — Option A is correct because the CloudFormation template creates a public subnet but does not set the 'MapPublicIpOnLaunch' property to 'true'. Without this setting, EC2 instances launched in the subnet do not automatically receive a public IPv4 address, which is required for internet-bound traffic to be routed through the Internet Gateway (IGW). Even if the route table has a default route (0.0.0.0/0) pointing to the IGW, the instance lacks a public IP, so the IGW cannot perform source NAT for outbound traffic, making internet access impossible.
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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