The answer is to configure the ALB to pass the X-Forwarded-For header and ensure backend logs parse it, as this is the standard, zero-overhead method for preserving client IP addresses behind an ALB. When an Application Load Balancer terminates TLS, it replaces the original source IP with its own private IP, so the client IP is inserted into the X-Forwarded-For header of the HTTP request; backend instances simply need to log this header value to capture the true client address. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this concept tests your understanding of how ALB handles HTTP/HTTPS traffic versus TCP traffic—a common trap is confusing Proxy Protocol (which adds overhead and is for TCP) with the X-Forwarded-For header (which is lightweight and native to HTTP). Remember: for HTTP behind an ALB, think “header, not proxy”—the X-Forwarded-For header is the built-in solution, while Proxy Protocol is an unnecessary extra for HTTP workloads.
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 designing a highly available architecture for a web application using an Application Load Balancer (ALB) across multiple Availability Zones. The ALB is internet-facing and uses TLS termination. The application requires that client IP addresses be preserved in the backend logs. The backend instances are in private subnets behind the ALB. Which configuration will ensure client IP addresses are preserved without additional overhead?
Attach a VPC Endpoint to the ALB to capture client IP
Why wrong: Incorrect; VPC Endpoints are for private access to AWS services, not for client IP preservation.
B
Enable Proxy Protocol v2 on the target group
Why wrong: Incorrect; Proxy Protocol is needed for TCP traffic, but ALB already preserves IP via X-Forwarded-For for HTTP/HTTPS.
C
Replace the ALB with a Network Load Balancer (NLB)
Why wrong: Incorrect; NLB preserves client IP for TCP/UDP, but the application uses HTTP, and switching to NLB would lose TLS termination and HTTP features.
D
Configure the ALB to pass the X-Forwarded-For header and ensure backend logs parse it
Correct; ALB automatically adds X-Forwarded-For header, and backend can log that to capture client IP.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Configure the ALB to pass the X-Forwarded-For header and ensure backend logs parse it
Option C is correct because ALB preserves the client IP address by inserting the X-Forwarded-For header, and enabling Proxy Protocol on the ALB is not necessary for HTTP/HTTPS traffic. Option A is wrong because enabling Proxy Protocol on the target group adds additional overhead and is typically used for TCP traffic. Option B is wrong because using Network Load Balancer would not preserve the client IP in the same way for HTTP; NLB preserves IP by default for TCP/UDP but not for HTTP. Option D is wrong because using a VPC Endpoint is irrelevant.
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.
✗
Attach a VPC Endpoint to the ALB to capture client IP
Why it's wrong here
Incorrect; VPC Endpoints are for private access to AWS services, not for client IP preservation.
✗
Enable Proxy Protocol v2 on the target group
Why it's wrong here
Incorrect; Proxy Protocol is needed for TCP traffic, but ALB already preserves IP via X-Forwarded-For for HTTP/HTTPS.
✗
Replace the ALB with a Network Load Balancer (NLB)
Why it's wrong here
Incorrect; NLB preserves client IP for TCP/UDP, but the application uses HTTP, and switching to NLB would lose TLS termination and HTTP features.
✓
Configure the ALB to pass the X-Forwarded-For header and ensure backend logs parse it
Why this is correct
Correct; ALB automatically adds X-Forwarded-For header, and backend can log that to capture client IP.
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.
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: Configure the ALB to pass the X-Forwarded-For header and ensure backend logs parse it — Option C is correct because ALB preserves the client IP address by inserting the X-Forwarded-For header, and enabling Proxy Protocol on the ALB is not necessary for HTTP/HTTPS traffic. Option A is wrong because enabling Proxy Protocol on the target group adds additional overhead and is typically used for TCP traffic. Option B is wrong because using Network Load Balancer would not preserve the client IP in the same way for HTTP; NLB preserves IP by default for TCP/UDP but not for HTTP. Option D is wrong because using a VPC Endpoint is irrelevant.
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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