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 CIDR of 10.0.0.0/16. They have two Availability Zones, each with a public subnet (10.0.1.0/24 and 10.0.2.0/24) and a private subnet (10.0.3.0/24 and 10.0.4.0/24). They have an internet-facing ALB in the public subnets and EC2 instances in the private subnets. The EC2 instances need to download updates from the internet. They deploy a NAT Gateway in each public subnet and add routes in the private subnet route tables pointing to the respective NAT Gateway in the same AZ. However, the EC2 instances in AZ2 cannot access the internet, while those in AZ1 can. 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.
The security group of the EC2 instances in AZ2 is blocking outbound traffic.
Why wrong: The issue is AZ-specific, not instance-specific.
B
The NAT Gateway in AZ2 does not have an Elastic IP address assigned.
A NAT Gateway requires an Elastic IP; without it, it cannot route traffic to the internet.
C
The private subnet in AZ2 is routing traffic to the NAT Gateway in AZ1, which is in a different Availability Zone and incurs cross-AZ charges but should still work.
Why wrong: Cross-AZ routing works, but the description says the route points to the NAT in the same AZ.
D
The route table for the private subnet in AZ2 is missing a route to the NAT Gateway.
Why wrong: According to the scenario, routes are added.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The NAT Gateway in AZ2 does not have an Elastic IP address assigned.
The NATGateway in AZ2 may be in a different public subnet than the one the private subnet route points to, or the NAT Gateway may not have an associated Elastic IP. Option A is wrong because cross-AZ routing is possible but not the issue. Option B is wrong because the route is to the specific NAT Gateway in the same AZ. Option D is wrong because the issue is specific to one AZ, not a global issue.
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 security group of the EC2 instances in AZ2 is blocking outbound traffic.
Why it's wrong here
The issue is AZ-specific, not instance-specific.
✓
The NAT Gateway in AZ2 does not have an Elastic IP address assigned.
Why this is correct
A NATGateway requires an Elastic IP; without it, it cannot route traffic to the internet.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
The private subnet in AZ2 is routing traffic to the NAT Gateway in AZ1, which is in a different Availability Zone and incurs cross-AZ charges but should still work.
Why it's wrong here
Cross-AZ routing works, but the description says the route points to the NAT in the same AZ.
✗
The route table for the private subnet in AZ2 is missing a route to the NAT Gateway.
Why it's wrong here
According to the scenario, routes are added.
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.
Trap categories for this question
Scenario analysis trap
According to the scenario, routes are added.
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 NAT Gateway in AZ2 does not have an Elastic IP address assigned. — The NAT Gateway in AZ2 may be in a different public subnet than the one the private subnet route points to, or the NAT Gateway may not have an associated Elastic IP. Option A is wrong because cross-AZ routing is possible but not the issue. Option B is wrong because the route is to the specific NAT Gateway in the same AZ. Option D is wrong because the issue is specific to one AZ, not a global issue.
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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