The answer is that the subnet lacks a route to the Internet Gateway because the route table is not associated with the subnet. In AWS, a subnet is private by default until you explicitly associate it with a route table that contains a default route (0.0.0.0/0) pointing to an Internet Gateway. Even if the Internet Gateway is attached to the VPC and the route table exists, traffic from the subnet cannot reach the internet without that association—the subnet simply has no path to the IGW. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this scenario tests your understanding of VPC routing fundamentals and the common misconfiguration where a CloudFormation template creates separate resources but omits the AWS::EC2::SubnetRouteTableAssociation resource. A frequent trap is assuming that creating a route table and an IGW automatically grants internet access to a subnet. Memory tip: "No association, no destination"—a subnet must be linked to a route table for any route to apply.
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 network engineer deploys this CloudFormation template. After deployment, an EC2 instance launched in PublicSubnet1 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 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 a route to the Internet Gateway because the route table is not associated with the subnet.
The most likely cause is that the route table containing the default route (0.0.0.0/0) pointing to the Internet Gateway is not associated with PublicSubnet1. Without this association, traffic from the subnet has no path to the Internet Gateway, even if the IGW is attached to the VPC. CloudFormation templates often create separate route tables and subnets, and a missing subnet-to-route-table association is a common misconfiguration.
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 Internet Gateway is not attached to the VPC.
Why it's wrong here
The AttachGateway resource attaches it.
✓
The subnet does not have a route to the Internet Gateway because the route table is not associated with the subnet.
Why this is correct
The template does not associate the custom route table with PublicSubnet1; it uses the main route table which lacks the IGW route.
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 subnet does not have MapPublicIpOnLaunch set to true.
Why it's wrong here
It is set to true.
✗
The security group is blocking outbound traffic.
Why it's wrong here
Security groups allow all outbound by default.
Common exam traps
Common exam trap: answer the scenario, not the keyword
AWS often tests the distinction between having an Internet Gateway attached to the VPC (which is necessary but not sufficient) and having a proper route table association with a default route to that IGW, leading candidates to mistakenly select Option A when the real issue is the missing subnet-to-route-table linkage.
Detailed technical explanation
How to think about this question
In AWS, a subnet is implicitly associated with the VPC's main route table unless explicitly associated with a custom route table. If the CloudFormation template creates a custom route table with a 0.0.0.0/0 route to the IGW but fails to associate it with PublicSubnet1, the subnet falls back to the main route table, which typically lacks an internet route. This is a common pitfall when using AWS::EC2::SubnetRouteTableAssociation resources — if the association is missing or misreferenced, the subnet remains isolated from the IGW.
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 a route to the Internet Gateway because the route table is not associated with the subnet. — The most likely cause is that the route table containing the default route (0.0.0.0/0) pointing to the Internet Gateway is not associated with PublicSubnet1. Without this association, traffic from the subnet has no path to the Internet Gateway, even if the IGW is attached to the VPC. CloudFormation templates often create separate route tables and subnets, and a missing subnet-to-route-table association is a common misconfiguration.
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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