The answer is that the internet gateway is not attached to the VPC. Without a formal attachment, the internet gateway exists as a disconnected resource, and the VPC’s route table cannot direct traffic to it, even if a default route (0.0.0.0/0) is present. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this scenario tests your understanding that an internet gateway must be explicitly attached to a VPC before it can function—a common oversight that traps candidates who focus only on route tables or security groups. The default network ACL allows all traffic, and the web server’s public IP and security group are correctly configured, so the missing attachment is the most likely cause. Remember the mnemonic: “IGW must be hitched to the VPC before traffic can ditch.”
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.
A company has a VPC with a public subnet hosting a web server. They want to make the web server accessible over the internet. The web server has a public IP address. The public subnet route table has a default route (0.0.0.0/0) to an internet gateway. The security group for the web server allows inbound HTTP (port 80) from 0.0.0.0/0. However, external users 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 the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The internet gateway is not attached to the VPC
The internet gateway must be attached to the VPC. If it is not attached, traffic cannot flow. Option B (NACL) could be blocking but default NACL allows all traffic. Option C (web server not listening) is possible but less likely than IGW attachment. Option D (route table) is already correct.
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.
✓
The internet gateway is not attached to the VPC
Why this is correct
Internet gateway must be attached for public traffic.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
The route table for the public subnet does not have a route to the internet gateway
Why it's wrong here
It does have the route.
✗
The network ACL for the public subnet is blocking inbound HTTP traffic
Why it's wrong here
Default NACL allows all traffic.
Common exam traps
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: The internet gateway is not attached to the VPC — The internet gateway must be attached to the VPC. If it is not attached, traffic cannot flow. Option B (NACL) could be blocking but default NACL allows all traffic. Option C (web server not listening) is possible but less likely than IGW attachment. Option D (route table) is already correct.
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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