The answer is that the source/destination check must be disabled on the NAT instance for traffic to be forwarded. By default, AWS EC2 instances verify that any traffic they send or receive has a source or destination IP matching the instance itself; this security feature prevents an instance from acting as a router. When a NAT instance is deployed to forward traffic from private subnets to the internet, it must handle packets where the source IP is not its own, so disabling this check is mandatory. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this concept frequently appears as a trick question where candidates overlook the default setting, assuming route tables alone solve the problem. A common trap is focusing on security groups or route propagation, but the core issue is that the instance’s network interface rejects non-matching traffic. Remember the mnemonic: “NAT needs no check” — if it’s a NAT, disable the source/destination check.
ANS-C01 Network Design Practice Question
This ANS-C01 practice question tests your understanding of network design. 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 multiple subnets. They want to ensure that all outbound traffic from EC2 instances in the VPC goes through a centralized NAT device for inspection. They have deployed a NAT instance in a public subnet and configured the route tables for private subnets to point to the NAT instance. However, traffic is not being routed through the NAT instance. What is the most likely cause?
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 source/destination check is enabled on the NAT instance.
Option C is correct because the NAT instance must have source/destination check disabled to forward traffic for other instances. Option A is wrong because the route table is already configured. Option B is wrong because the NAT instance itself can have its own route table for internet access. Option D is wrong because security groups can allow traffic; the issue is routing.
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.
✓
The source/destination check is enabled on the NAT instance.
Why this is correct
By default, EC2 instances check that the source or destination of traffic matches their own IP. This must be disabled for NAT instances.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
The NAT instance does not have a public IP address.
Why it's wrong here
A NAT instance needs a public IP for internet access, but the question implies it is in a public subnet.
✗
The route table for the private subnets does not have a default route (0.0.0.0/0) pointing to the NAT instance.
Why it's wrong here
The description says the route tables are configured, so this is not the issue.
✗
The security group of the NAT instance blocks outbound traffic.
Why it's wrong here
While possible, the most common cause is source/destination check.
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 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 ANS-C01 subnetting questions on CIDR, address ranges, and subnet selection.
Network Design — This question tests Network Design — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: The source/destination check is enabled on the NAT instance. — Option C is correct because the NAT instance must have source/destination check disabled to forward traffic for other instances. Option A is wrong because the route table is already configured. Option B is wrong because the NAT instance itself can have its own route table for internet access. Option D is wrong because security groups can allow traffic; the issue is routing.
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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