The answer is asymmetric routing caused by the EC2 instances’ default route to the NAT Gateway. When the Network Load Balancer uses client IP preservation, the target instances see the original client source IP, not the NLB’s private IP. The instances then attempt to send response traffic directly to that client IP, but their default route points to the NAT Gateway, which forwards the response out to the internet. The client, however, expects the reply to come from the NLB’s IP address, so the return path is broken—this asymmetry causes the intermittent timeouts. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this scenario tests your understanding of how NLB client IP preservation interacts with routing tables and NAT gateways. A common trap is assuming security groups or health checks are the issue, but NLB target groups don’t use security groups, and health checks are unaffected. Memory tip: “Preserve the client IP, but preserve the return path too—if the route goes NAT, the client gets a spat.”
ANS-C01 Network Implementation Practice Question
This ANS-C01 practice question tests your understanding of network implementation. Match the stated requirement to the specific cloud service, access model, or configuration option — many options are valid in isolation but not for this scenario. 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 CIDR 10.0.0.0/16. They have a public subnet (10.0.1.0/24) and a private subnet (10.0.2.0/24). They have a Network Load Balancer (NLB) in the public subnet, and EC2 instances in the private subnet. The NLB has a target group pointing to the EC2 instances. The security group for the EC2 instances allows traffic from the NLB's private IP addresses on port 80. The network ACL for the private subnet allows inbound traffic on port 80 from the public subnet CIDR and outbound ephemeral ports to 0.0.0.0/0. However, clients connecting to the NLB experience intermittent timeouts. The network engineer checks the NLB logs and sees that connections are established but occasionally drop. The engineer also notices that the EC2 instances have a default route to a NAT Gateway in the public subnet. What is the most likely cause of the intermittent timeouts?
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
✓
The EC2 instances have a default route to the NAT Gateway, causing asymmetric routing.
Option A is correct. When the NLB uses client IP preservation (default), the target instance receives traffic with the client's source IP. The target then sends the response back to the client IP. But the target's default route points to the NAT Gateway, so the response goes to the NAT Gateway instead of directly back to the NLB. The NAT Gateway then sends the response to the internet, but the client expects the response from the NLB's IP, causing asymmetry and potential drops. Option B is wrong because health checks are independent. Option C is wrong because disabling cross-zone load balancing does not cause timeouts. Option D is wrong because the NLB does not have security groups.
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.
✗
Cross-zone load balancing is disabled on the NLB.
Why it's wrong here
This would cause uneven distribution but not timeouts.
✗
The NLB health checks are failing, causing targets to be removed.
Why it's wrong here
This would cause complete failure, not intermittent.
✓
The EC2 instances have a default route to the NAT Gateway, causing asymmetric routing.
Why this is correct
Return traffic goes to NAT instead of NLB, breaking the connection.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
The security group for the NLB is blocking inbound traffic.
Why it's wrong here
NLBs do not have security groups.
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 healthcare organisation deploys an application with a public-facing web tier and a private database tier. The database subnet has no public IP and only accepts connections from the web tier's security group. Questions like this test whether you can design cloud network isolation using VNets/VPCs, subnets, and security group rules.
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: The EC2 instances have a default route to the NAT Gateway, causing asymmetric routing. — Option A is correct. When the NLB uses client IP preservation (default), the target instance receives traffic with the client's source IP. The target then sends the response back to the client IP. But the target's default route points to the NAT Gateway, so the response goes to the NAT Gateway instead of directly back to the NLB. The NAT Gateway then sends the response to the internet, but the client expects the response from the NLB's IP, causing asymmetry and potential drops. Option B is wrong because health checks are independent. Option C is wrong because disabling cross-zone load balancing does not cause timeouts. Option D is wrong because the NLB does not have security groups.
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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