- A
Deploy a NAT gateway in each public subnet and configure private route tables with a default route to the NAT gateway in the same AZ.
Provides high availability (each AZ independent) and uses managed service, cost-effective for moderate traffic.
- B
Create a single NAT gateway in one public subnet and route all private traffic to it.
Why wrong: Single point of failure; if that AZ fails, private instances lose internet access.
- C
Attach an internet gateway to the VPC and add a default route to it in the private subnets.
Why wrong: Private subnets need a NAT device, not direct internet gateway access.
- D
Launch a NAT instance in each public subnet and configure the private route tables.
Why wrong: NAT instances require manual scaling and patching, less reliable than managed NAT gateways.
Quick Answer
The most cost-effective and highly available NAT gateway configuration for private subnets is to deploy a NAT gateway in each public subnet and configure private route tables with a default route to the NAT gateway in the same Availability Zone. This design ensures that if one AZ fails, private instances in the other AZ retain internet access through their local NAT gateway, avoiding a single point of failure while eliminating the management overhead and variable costs of NAT instances. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this scenario tests your understanding of high availability versus cost trade-offs in VPC networking, often appearing as a trap where candidates choose a single NAT gateway to save money, forgetting that it creates a single point of failure. A common memory tip is “one NAT per AZ for HA on the cheap”—think of it as distributing the cost across zones to maintain resilience without paying for idle capacity in a multi-AZ active-passive setup.
ANS-C01 Network Implementation Practice Question
This ANS-C01 practice question tests your understanding of network implementation. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. 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 deploying a VPC with public and private subnets in two Availability Zones. The public subnets are used for NAT gateways and an Application Load Balancer (ALB). The private subnets host EC2 instances running a web application. What is the most cost-effective and highly available configuration for internet access from the private instances?
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
Deploy a NAT gateway in each public subnet and configure private route tables with a default route to the NAT gateway in the same AZ.
Option D is correct because using a NAT gateway in each AZ provides high availability and cost-effectiveness compared to NAT instances (which require management) or a single NAT gateway (single point of failure). Option A is wrong because NAT instances are less reliable and require maintenance. Option B is wrong because a single NAT gateway lacks high availability. Option C is wrong because internet gateway is not used for private instances; it's for public subnets.
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.
- ✓
Deploy a NAT gateway in each public subnet and configure private route tables with a default route to the NAT gateway in the same AZ.
Why this is correct
Provides high availability (each AZ independent) and uses managed service, cost-effective for moderate traffic.
Related concept
CIDR notation defines the prefix length.
- ✗
Create a single NAT gateway in one public subnet and route all private traffic to it.
Why it's wrong here
Single point of failure; if that AZ fails, private instances lose internet access.
- ✗
Attach an internet gateway to the VPC and add a default route to it in the private subnets.
- ✗
Launch a NAT instance in each public subnet and configure the private route tables.
Why it's wrong here
NAT instances require manual scaling and patching, less reliable than managed NAT gateways.
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 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.
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FAQ
Questions learners often ask
What does this ANS-C01 question test?
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: Deploy a NAT gateway in each public subnet and configure private route tables with a default route to the NAT gateway in the same AZ. — Option D is correct because using a NAT gateway in each AZ provides high availability and cost-effectiveness compared to NAT instances (which require management) or a single NAT gateway (single point of failure). Option A is wrong because NAT instances are less reliable and require maintenance. Option B is wrong because a single NAT gateway lacks high availability. Option C is wrong because internet gateway is not used for private instances; it's for public subnets.
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.
What is the key concept behind this question?
CIDR notation defines the prefix length.
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Last reviewed: Jun 20, 2026
This ANS-C01 practice question is part of Courseiva's free Amazon Web Services certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the ANS-C01 exam.
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