The answer is that the route table for the public subnet 10.0.1.0/24 does not have a route to the Internet Gateway. This is the most likely cause because the NAT Gateway, which resides in that public subnet, must have a direct path to the Internet Gateway to translate and forward traffic from the private subnet; without that route, the NAT Gateway itself cannot reach the internet, even though the private subnets’ route tables correctly point 0.0.0.0/0 to the NAT Gateway. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this scenario tests your understanding that a NAT Gateway is not a transit device—it requires its own subnet’s route table to have an Internet Gateway route, a common trap where candidates focus only on the private subnet routing. Remember the memory tip: “NAT needs a gateway to the gate—if the public subnet lacks an IGW route, the NAT can’t communicate out.”
ANS-C01 Network Security, Compliance and Governance Practice Question
This ANS-C01 practice question tests your understanding of network security, compliance and governance. 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 CIDR 10.0.0.0/16. They have public subnets (10.0.1.0/24, 10.0.2.0/24) and private subnets (10.0.3.0/24, 10.0.4.0/24) in two Availability Zones. An Internet Gateway is attached to the VPC, and a NAT Gateway is in public subnet 10.0.1.0/24. The private subnets route 0.0.0.0/0 to the NAT Gateway. The security team notices that instances in private subnet 10.0.3.0/24 can reach the internet, but instances in private subnet 10.0.4.0/24 cannot. Both private subnets have the same route table configuration. The network ACLs for both private subnets are set to allow all inbound and outbound traffic. What is the most likely cause of the issue?
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 route table for the public subnet 10.0.1.0/24 does not have a route to the Internet Gateway.
Option B is correct because if the NATGateway is in subnet 10.0.1.0/24, the route table for that subnet must have a route to the Internet Gateway for the NAT Gateway to work. Without that route, the NAT Gateway cannot send traffic to the internet. Option A is wrong because the route tables for private subnets are correct. Option C is wrong because the network ACLs allow all traffic. Option D is wrong because security groups on the instances would affect both subnets equally.
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 route table for the public subnet 10.0.1.0/24 does not have a route to the Internet Gateway.
Why this is correct
The NATGateway needs a route to the Internet Gateway through its subnet's route table.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
The security group on the instances in private subnet 10.0.4.0/24 is blocking outbound traffic.
Why it's wrong here
Security groups would affect both subnets similarly if applied.
✗
The network ACL for private subnet 10.0.4.0/24 is blocking outbound traffic.
Why it's wrong here
Network ACLs are set to allow all traffic.
✗
The route table for private subnet 10.0.4.0/24 does not have a route to the NAT Gateway.
Why it's wrong here
Both private subnets share the same route table, so they have the same route.
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.
Trap categories for this question
Similar concept trap
Security groups would affect both subnets similarly if applied.
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
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.
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 Security, Compliance and Governance — This question tests Network Security, Compliance and Governance — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: The route table for the public subnet 10.0.1.0/24 does not have a route to the Internet Gateway. — Option B is correct because if the NAT Gateway is in subnet 10.0.1.0/24, the route table for that subnet must have a route to the Internet Gateway for the NAT Gateway to work. Without that route, the NAT Gateway cannot send traffic to the internet. Option A is wrong because the route tables for private subnets are correct. Option C is wrong because the network ACLs allow all traffic. Option D is wrong because security groups on the instances would affect both subnets equally.
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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