The correct answer is to ensure the NLB is configured with a subnet from each Availability Zone and the targets are in the same subnets, because AWS Network Load Balancer preserves client source IP addresses by default at layer 4, meaning no special configuration like proxy protocol is required. This default behavior occurs because the NLB forwards packets directly to the backend instances without performing Network Address Translation (NAT) on the source IP, so the EC2 instances see the original client IP in their access logs. On the AWS Certified Security Specialty SCS-C02 exam, this concept tests your understanding of how NLB differs from Application Load Balancer, which does replace the source IP with its own private IP. A common trap is assuming you need to enable proxy protocol or stickiness, but for NLB, source IP preservation is inherent when targets reside in the same subnets as the NLB’s elastic network interfaces. Memory tip: “NLB = No NAT, so the client IP stays intact.”
SCS-C02 Infrastructure Security Practice Question
This SCS-C02 practice question tests your understanding of infrastructure security. 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 uses Network Load Balancer (NLB) in front of a fleet of EC2 instances in private subnets. Security team requires that the source IP addresses of clients be preserved in the access logs of the backend instances. Which configuration should the security engineer verify?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Ensure the NLB is configured with a subnet from each Availability Zone and the targets are in the same subnets
NLB preserves source IP by default, so no special configuration is needed. Option C is correct. Option A is wrong because NLB uses the client IP directly. Option B is wrong because proxy protocol is an alternative but not required for NLB. Option D is wrong because target group stickiness does not affect source IP preservation.
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.
✗
Configure proxy protocol v2 on the target group
Why it's wrong here
Proxy protocol is not required for NLB; NLB preserves source IP by default.
✓
Ensure the NLB is configured with a subnet from each Availability Zone and the targets are in the same subnets
Why this is correct
NLB preserves source IP by default; no special configuration is needed.
Stickiness does not affect source IP preservation.
✗
Enable cross-zone load balancing on the NLB
Why it's wrong here
Cross-zone load balancing does not affect source IP preservation.
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 SCS-C02 subnetting questions on CIDR, address ranges, and subnet selection.
Infrastructure Security — This question tests Infrastructure Security — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: Ensure the NLB is configured with a subnet from each Availability Zone and the targets are in the same subnets — NLB preserves source IP by default, so no special configuration is needed. Option C is correct. Option A is wrong because NLB uses the client IP directly. Option B is wrong because proxy protocol is an alternative but not required for NLB. Option D is wrong because target group stickiness does not affect source IP preservation.
What should I do if I get this SCS-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 SCS-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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Question Discussion
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