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 is designing a VPC with public and private subnets in two Availability Zones. The private subnets host EC2 instances that need to download patches from the internet. The company wants to minimize costs while ensuring high availability. Which solution meets these requirements?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "minimum / minimize"
Why it matters: Asks for the least resource use — fewest addresses, smallest subnet, lowest overhead. Eliminate over-provisioned options even if they would technically work.
Create a NAT gateway in one public subnet and use a second NAT gateway in a different AZ as a backup, but only route traffic to the primary.
Why wrong: Traffic does not automatically failover; requires manual intervention.
B
Create a NAT gateway in one public subnet and configure route tables for private subnets to use it.
Why wrong: Single point of failure.
C
Launch a NAT instance in one public subnet and configure route tables for private subnets to use it.
Why wrong: NAT instance is not highly available and requires manual failover.
D
Create a NAT gateway in a public subnet in each Availability Zone and configure route tables for private subnets to use the NAT gateway in the same AZ.
Highly available and cost-effective with proper AZ-level routing.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Create a NAT gateway in a public subnet in each Availability Zone and configure route tables for private subnets to use the NAT gateway in the same AZ.
Option D is correct because two NAT gateways (one per AZ) provide high availability, and each is placed in a public subnet. Option A is wrong because a single NAT gateway is a single point of failure. Option B is wrong because NAT instances are less reliable and require more management. Option C is wrong because a NAT gateway in one AZ provides no cross-AZ high availability if that AZ fails.
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.
✗
Create a NAT gateway in one public subnet and use a second NAT gateway in a different AZ as a backup, but only route traffic to the primary.
Why it's wrong here
Traffic does not automatically failover; requires manual intervention.
✗
Create a NAT gateway in one public subnet and configure route tables for private subnets to use it.
Why it's wrong here
Single point of failure.
✗
Launch a NAT instance in one public subnet and configure route tables for private subnets to use it.
Why it's wrong here
NAT instance is not highly available and requires manual failover.
✓
Create a NAT gateway in a public subnet in each Availability Zone and configure route tables for private subnets to use the NAT gateway in the same AZ.
Why this is correct
Highly available and cost-effective with proper AZ-level routing.
Clue confirmation
The clue word "minimum / minimize" in the question point toward this answer.
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 startup's cloud architect reviews their monthly bill and notices costs are higher than expected for a long-running batch job. Switching from on-demand instances to Reserved Instances — or using Spot/Preemptible VMs — can reduce compute costs by up to 72 %. Questions like this test whether you understand the tradeoffs between commitment, flexibility, and cost across cloud pricing models.
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: Create a NAT gateway in a public subnet in each Availability Zone and configure route tables for private subnets to use the NAT gateway in the same AZ. — Option D is correct because two NAT gateways (one per AZ) provide high availability, and each is placed in a public subnet. Option A is wrong because a single NAT gateway is a single point of failure. Option B is wrong because NAT instances are less reliable and require more management. Option C is wrong because a NAT gateway in one AZ provides no cross-AZ high availability if that AZ fails.
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: "minimum / minimize". Asks for the least resource use — fewest addresses, smallest subnet, lowest overhead. Eliminate over-provisioned options even if they would technically work.
What is the key concept behind this question?
CIDR notation defines the prefix length.
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