The answer is that the EC2 instance needs an Elastic IP address assigned to it or the subnet must have auto-assign public IP enabled. This is because an instance in a public subnet with a route to an Internet Gateway still requires a public IPv4 address for bidirectional internet traffic; without it, the instance can initiate outbound connections through the IGW’s NAT, but inbound traffic has no routable destination. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this scenario tests your understanding that a public subnet alone does not guarantee internet accessibility—the instance must have a publicly reachable IP. A common trap is assuming a security group or NACL is the culprit when the real issue is the missing public IP assignment. Memory tip: “Route to IGW gets you out, but a public IP lets them in.”
ANS-C01 Network Implementation Practice Question
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.
Refer to the exhibit. A CloudFormation template creates a VPC with public and private subnets. The template includes an Internet Gateway and a route table with a default route to the IGW, associated with the public subnet. An EC2 instance launched in the public subnet cannot be reached from the internet. The security group allows inbound HTTP from 0.0.0.0/0. What is the MOST likely missing resource?
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
✓
An Elastic IP address assigned to the instance or auto-assign public IP enabled on the subnet
Option C is correct because the template does not define a route table or route for the private subnet. However, the instance is in the public subnet, so the issue is that the public subnet might not have the route table associated. But looking at the template, it has PublicSubnetRouteTableAssociation. The missing piece is that the instance likely needs an Elastic IP or auto-assign public IP. Option A is not needed for public subnets with IGW. Option B is not needed for public subnets. Option D is not needed because the instance is in a public subnet.
Key principle: Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
Common exam trap: usable hosts are not the same as total addresses
Subnetting questions often tempt you into counting all addresses. In normal IPv4 subnets, the network and broadcast addresses are not usable host addresses.
Detailed technical explanation
How to think about this question
Subnetting questions test whether you can identify the network, broadcast address, usable range, mask and correct subnet. Slow down enough to calculate the block size correctly.
KKey Concepts to Remember
CIDR notation defines the prefix length.
Block size helps identify subnet boundaries.
Network and broadcast addresses are not usable hosts in normal IPv4 subnets.
The required host count determines the smallest suitable subnet.
TExam Day Tips
→Write the block size before choosing the subnet.
→Check whether the question asks for hosts, subnets or a specific address range.
→Do not confuse /24, /25, /26 and /27 host counts.
Key takeaway
Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.
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.
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related ANS-C01 subnetting questions on CIDR, address ranges, and subnet selection.
Network Implementation — This question tests Network Implementation — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: An Elastic IP address assigned to the instance or auto-assign public IP enabled on the subnet — Option C is correct because the template does not define a route table or route for the private subnet. However, the instance is in the public subnet, so the issue is that the public subnet might not have the route table associated. But looking at the template, it has PublicSubnetRouteTableAssociation. The missing piece is that the instance likely needs an Elastic IP or auto-assign public IP. Option A is not needed for public subnets with IGW. Option B is not needed for public subnets. Option D is not needed because the instance is in a public subnet.
What should I do if I get this ANS-C01 question wrong?
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related ANS-C01 subnetting questions on CIDR, address ranges, and subnet selection.
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?
CIDR notation defines the prefix length.
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Question Discussion
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