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 public subnet containing a NAT gateway and a private subnet containing Amazon EC2 instances. The instances in the private subnet need to download patches from the internet. The NAT gateway is in the public subnet and has an Elastic IP address. The private subnet's route table has a default route pointing to the NAT gateway. However, the instances 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 public subnet's route table does not have a route to an internet gateway.
Option D is correct. For a NATgateway to work, it must be in a public subnet with a route to an internet gateway. If the public subnet does not have a route to the internet gateway, the NAT gateway cannot forward traffic to the internet. Option A is incorrect because security groups for instances in private subnets should allow outbound traffic, but that is not the most likely cause. Option B is incorrect because the NAT gateway automatically replies to traffic; no route back is needed for responses. Option C is incorrect because the NAT gateway does not need a security group; it uses network ACLs and route tables.
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 public subnet's route table does not have a route to an internet gateway.
Why this is correct
The public subnet must have a route to an internet gateway for the NAT gateway to reach the internet.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
The security group attached to the EC2 instances does not allow outbound traffic to the internet.
Why it's wrong here
Security groups are stateful; outbound traffic is allowed by default unless explicitly denied.
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 Design — This question tests Network Design — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: The public subnet's route table does not have a route to an internet gateway. — Option D is correct. For a NAT gateway to work, it must be in a public subnet with a route to an internet gateway. If the public subnet does not have a route to the internet gateway, the NAT gateway cannot forward traffic to the internet. Option A is incorrect because security groups for instances in private subnets should allow outbound traffic, but that is not the most likely cause. Option B is incorrect because the NAT gateway automatically replies to traffic; no route back is needed for responses. Option C is incorrect because the NAT gateway does not need a security group; it uses network ACLs and route tables.
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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