The correct answer is to use a Network Load Balancer with Elastic IP addresses. This works because an NLB supports static IP assignment per Availability Zone through Elastic IPs, giving clients a fixed set of addresses to whitelist, whereas an Application Load Balancer’s DNS name resolves to dynamic IPs that can change over time. On the AWS Certified SAP on AWS Specialty PAS-C01 exam, this scenario tests your understanding of the architectural trade-off between ALB and NLB when static IPs are a non-negotiable requirement for client whitelisting. A common trap is assuming an ALB can be fronted by a NAT gateway or use a global accelerator for static IPs, but the simplest, most direct solution is an NLB with EIPs. Memory tip: think “NLB = Nice, Locked, Block” — it locks down a fixed block of IPs for whitelisting, while ALB is “Always Changing.”
PAS-C01 Migration Practice Question
This PAS-C01 practice question tests your understanding of migration. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. 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 migrating a legacy on-premises application to AWS. The application requires static IP addresses for its clients to whitelist. The migration strategy includes using an Application Load Balancer (ALB) to distribute traffic across multiple EC2 instances in an Auto Scaling group. The clients require a fixed set of IP addresses to whitelist. Which solution meets the requirement?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Use a Network Load Balancer with Elastic IP addresses.
Option C is correct because a Network Load Balancer (NLB) supports static IP addresses via Elastic IP (EIP) assignment per Availability Zone, which allows clients to whitelist a fixed set of IPs. The ALB, by contrast, does not support static IP assignment; its DNS name resolves to dynamic IPs that can change. Using an NLB with EIPs meets the requirement for a fixed whitelist while still distributing traffic to the ALB or directly to EC2 instances.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
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 AWS Global Accelerator with the Application Load Balancer as an endpoint.
Why it's wrong here
Global Accelerator provides static IPs but is overkill and more expensive for this simple requirement.
✗
Use an Application Load Balancer and assign Elastic IP addresses to it.
Why it's wrong here
ALB does not support Elastic IP assignment; it uses dynamic IP addresses.
✓
Use a Network Load Balancer with Elastic IP addresses.
Why this is correct
NLB supports static IP addresses via Elastic IPs, meeting the whitelisting requirement.
Related concept
Read the scenario before looking for a memorised answer.
✗
Launch EC2 instances with Elastic IPs and place them behind a NAT Gateway.
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates assume an Application Load Balancer can be assigned Elastic IPs because they are familiar with assigning EIPs to EC2 instances, but ALBs do not support static IP assignment at all, making the NLB the only correct choice for fixed whitelist IPs.
Detailed technical explanation
How to think about this question
Under the hood, an NLB operates at Layer 4 and preserves the client IP address by default, which is critical for whitelisting scenarios where the backend needs to see the original client IP. When you assign an Elastic IP to an NLB, it is bound to the NLB's network interface per Availability Zone, ensuring that the IP remains static even if the underlying instances scale or fail. In contrast, an ALB operates at Layer 7 and uses a DNS-based routing mechanism that resolves to multiple IP addresses that can change over time, making it unsuitable for static IP whitelisting.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
TExam Day Tips
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
An e-commerce site experiences heavy traffic on Black Friday and near-zero traffic during off-peak weeks. Rather than provisioning permanent large VMs, the team uses auto-scaling groups that add capacity automatically under load and reduce it overnight. Questions like this test whether you understand elasticity, availability zones, and cloud compute scaling patterns.
What to study next
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Migration — This question tests Migration — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Use a Network Load Balancer with Elastic IP addresses. — Option C is correct because a Network Load Balancer (NLB) supports static IP addresses via Elastic IP (EIP) assignment per Availability Zone, which allows clients to whitelist a fixed set of IPs. The ALB, by contrast, does not support static IP assignment; its DNS name resolves to dynamic IPs that can change. Using an NLB with EIPs meets the requirement for a fixed whitelist while still distributing traffic to the ALB or directly to EC2 instances.
What should I do if I get this PAS-C01 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Question Discussion
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