The correct answer is to use a single NAT Gateway with an Elastic IP for a fixed source address, but this creates a single point of failure; for true high availability with fixed IPs, you must accept that a failed NAT Gateway in one AZ will break connectivity for instances in that AZ unless you use a solution like an AWS Transit Gateway or a centralized NAT appliance. The core technical concept is that a NAT Gateway’s Elastic IP provides a fixed public IP for whitelisting, but each NAT Gateway is confined to a single Availability Zone. To maintain both high availability and a fixed IP, you would need to route all traffic through a single NAT Gateway, which sacrifices fault tolerance, or use multiple NAT Gateways and accept that the source IP changes upon failover—there is no native AWS service that provides both a fixed IP and automatic cross-AZ failover for NAT. On the SOA-C02 exam, this question tests your understanding of the trade-off between fixed IP requirements and high availability architecture, often trapping candidates who assume a Network Load Balancer or multiple NAT Gateways can preserve a single source IP. A helpful memory tip: “One NAT, one IP, one zone—if it fails, you’re alone.”
SOA-C02 Networking and Content Delivery Practice Question
This SOA-C02 practice question tests your understanding of networking and content delivery. 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 public and private subnets across two Availability Zones. They have a NAT Gateway in each public subnet for high availability. EC2 instances in the private subnets need to access an external service that requires a fixed IP address for whitelisting. The administrator configured the route tables to use the NAT Gateway in the same Availability Zone. However, when the NAT Gateway in AZ-A fails, instances in AZ-A lose internet connectivity. What should the administrator do to ensure high availability with fixed IP addresses?
Use a single NAT Gateway with an Elastic IP and create a route table with a route to the NAT Gateway for all private subnets, but also configure a second NAT Gateway in another AZ and update route tables to failover
For fixed IP, use a single NAT Gateway; but for HA, use multiple and use a routing failover mechanism like using a Lambda to update route tables.
B
Configure a proxy server in the private subnets and route traffic through it
Why wrong: A proxy would still need a NAT Gateway.
C
Replace the NAT Gateways with an Internet Gateway in each public subnet
Why wrong: Internet Gateways make subnets public, not private.
D
Use a VPN connection to the external service instead of the internet
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Use a single NAT Gateway with an Elastic IP and create a route table with a route to the NAT Gateway for all private subnets, but also configure a second NAT Gateway in another AZ and update route tables to failover
Option D is correct because using a single NATGateway with a Network Load Balancer would not provide a fixed IP; a NAT Gateway provides an Elastic IP. For high availability, you can use multiple NAT Gateways and configure route tables to failover, but that changes the source IP. Using a NAT Gateway per AZ with route tables pointing to the local NAT Gateway is the best practice for HA. However, if a NAT Gateway fails, instances in that AZ lose internet. To maintain fixed IPs, the administrator could use a single NAT Gateway but that is a single point of failure. Option A is wrong because an Internet Gateway is not a NAT device. Option B is wrong because a VPN does not provide internet access. Option C is wrong because a proxy would have the same 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.
✓
Use a single NAT Gateway with an Elastic IP and create a route table with a route to the NAT Gateway for all private subnets, but also configure a second NAT Gateway in another AZ and update route tables to failover
Why this is correct
For fixed IP, use a single NATGateway; but for HA, use multiple and use a routing failover mechanism like using a Lambda to update route tables.
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 SOA-C02 subnetting questions on CIDR, address ranges, and subnet selection.
Networking and Content Delivery — This question tests Networking and Content Delivery — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: Use a single NAT Gateway with an Elastic IP and create a route table with a route to the NAT Gateway for all private subnets, but also configure a second NAT Gateway in another AZ and update route tables to failover — Option D is correct because using a single NAT Gateway with a Network Load Balancer would not provide a fixed IP; a NAT Gateway provides an Elastic IP. For high availability, you can use multiple NAT Gateways and configure route tables to failover, but that changes the source IP. Using a NAT Gateway per AZ with route tables pointing to the local NAT Gateway is the best practice for HA. However, if a NAT Gateway fails, instances in that AZ lose internet. To maintain fixed IPs, the administrator could use a single NAT Gateway but that is a single point of failure. Option A is wrong because an Internet Gateway is not a NAT device. Option B is wrong because a VPN does not provide internet access. Option C is wrong because a proxy would have the same issue.
What should I do if I get this SOA-C02 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 SOA-C02 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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