The answer is to create a NAT Gateway in each Availability Zone. This is required because a single NAT Gateway represents a critical single point of failure; if the AZ hosting that gateway goes down, all private instances in the other AZ lose outbound internet access, breaking the application’s ability to reach patches, licensing servers, or AWS APIs. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this scenario tests your understanding of designing a fault-tolerant VPC architecture that survives an AZ failure—a core objective of the exam’s high availability domain. A common trap is assuming that a NAT Gateway is inherently highly available, but it is actually zonal, not regional. The ALB and Multi-AZ RDS already provide AZ resilience, so the missing piece is ensuring each private subnet has its own NAT Gateway in its respective AZ, with corresponding route table entries pointing to that local gateway. Memory tip: “One NAT per AZ keeps the traffic flowing when an AZ goes away.”
ANS-C01 Network Design Practice Question
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 is deploying a new application on AWS and needs a highly available architecture across two Availability Zones (AZs) in a single region. The application consists of an Application Load Balancer (ALB) in front of a fleet of EC2 instances running in an Auto Scaling group, and an Amazon RDS for MySQL database with Multi-AZ deployment. The company requires that the application remain available even if an entire AZ fails. The network team has designed the VPC with two public subnets and two private subnets, each in a different AZ. The ALB is internet-facing and placed in the public subnets. The EC2 instances are in the private subnets. The RDS instance is also in the private subnets. The route tables are configured with a default route via an Internet Gateway for public subnets and via a NAT Gateway for private subnets. What change is MOST likely needed to ensure the architecture can survive an AZ failure?
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
✓
Create a NAT Gateway in each Availability Zone
To survive an AZ failure, each AZ must have its own NATGateway to provide outbound internet access for instances in that AZ. If a single NAT Gateway is used (e.g., in one AZ), and that AZ fails, instances in the other AZ lose internet connectivity. Similarly, a single NAT Gateway would be a single point of failure. Therefore, the architecture should include a NAT Gateway in each AZ. The ALB and RDS Multi-AZ already provide AZ resilience. So the missing component is a NAT Gateway per AZ.
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.
✗
Add a second Internet Gateway in the other AZ
Why it's wrong here
One Internet Gateway per VPC is sufficient; it is a highly available service.
✗
Use a Network Load Balancer instead of an Application Load Balancer
Why it's wrong here
ALB is already highly available across AZs; NLB is not needed for this issue.
✓
Create a NAT Gateway in each Availability Zone
Why this is correct
A NATGateway per AZ ensures outbound connectivity survives an AZ failure.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Configure the Auto Scaling group to launch instances in both AZs
Why it's wrong here
This is already likely configured; the question implies it's set up.
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: Create a NAT Gateway in each Availability Zone — To survive an AZ failure, each AZ must have its own NAT Gateway to provide outbound internet access for instances in that AZ. If a single NAT Gateway is used (e.g., in one AZ), and that AZ fails, instances in the other AZ lose internet connectivity. Similarly, a single NAT Gateway would be a single point of failure. Therefore, the architecture should include a NAT Gateway in each AZ. The ALB and RDS Multi-AZ already provide AZ resilience. So the missing component is a NAT Gateway per AZ.
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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