The answer is a missing inbound rule for ephemeral ports in the network ACL of the public subnet. This is correct because NACLs are stateless, meaning they evaluate inbound and outbound traffic independently; while the outbound rule allows the NAT gateway to send traffic out, the return traffic from the internet arrives on a high-numbered ephemeral port, and if the inbound NACL rule does not explicitly allow that traffic, it is dropped. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this scenario tests your understanding of stateless versus stateful filtering and how NAT gateway traffic flows through a public subnet—a common trap is assuming the NAT gateway’s state or the route table is the issue, when in fact the problem lies in the NACL blocking return traffic. Remember the mnemonic: NAT needs NACL love both ways—outbound for the request, inbound for the reply on ephemeral ports.
ANS-C01 Network Implementation Practice Question
This ANS-C01 practice question tests your understanding of network implementation. 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 private subnets that use a NAT gateway for outbound internet access. The NAT gateway is in a public subnet with an Elastic IP. Users report that some applications are failing to connect to external services. Network engineers confirm that the NAT gateway is in the 'available' state and the route tables have a default route (0.0.0.0/0) pointing to the NAT gateway. 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 network ACL for the public subnet is blocking inbound traffic on ephemeral ports.
Option C is correct because NACLs are stateless and must allow both inbound and outbound traffic for the ephemeral ports used by the NATgateway. If the inbound rule for ephemeral ports is missing, return traffic is blocked. Option A is wrong because the route table is correct. Option B is wrong because NAT gateway can handle multiple connections. Option D is wrong because the problem is inbound return traffic, not outbound.
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 security group for the private instances is blocking outbound traffic.
Why it's wrong here
Outbound traffic is allowed by default; the issue is return traffic.
✓
The network ACL for the public subnet is blocking inbound traffic on ephemeral ports.
Why this is correct
NACL must allow inbound traffic from the internet on ephemeral ports (1024-65535) for return traffic.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
NATgateway supports up to 55,000 simultaneous connections, unlikely cause.
✗
The route table for the private subnets is missing a route to the internet gateway.
Why it's wrong here
Private subnets route to NATgateway, not internet gateway.
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 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 network ACL for the public subnet is blocking inbound traffic on ephemeral ports. — Option C is correct because NACLs are stateless and must allow both inbound and outbound traffic for the ephemeral ports used by the NAT gateway. If the inbound rule for ephemeral ports is missing, return traffic is blocked. Option A is wrong because the route table is correct. Option B is wrong because NAT gateway can handle multiple connections. Option D is wrong because the problem is inbound return traffic, not outbound.
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.
About these practice questions
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These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. A company has a VPC with public and private subnets. They have a NAT gateway in a public subnet. They want to provide internet access to instances in private subnets. The NAT gateway is configured with an Elastic IP. The private instances still cannot access the internet. The route table for the private subnets has a default route (0.0.0.0/0) pointing to the NAT gateway. What is missing?
medium
A.The network ACL for the private subnet allows outbound traffic
B.The private subnet route table is missing a route to the NAT gateway
C.The security group for the instances allows outbound traffic
✓ D.The route table for the public subnet where the NAT gateway resides does not have a default route to an internet gateway
Why D: Option C is correct because the NAT gateway must be in a public subnet with a route table that has a default route to an internet gateway. Option A is wrong because NACLs are usually permissive. Option B is wrong because security groups are stateful. Option D is wrong because the default route is already set.
Variation 2. A company has a VPC with public and private subnets. They have a NAT gateway in a public subnet for outbound internet access from private instances. Which TWO of the following are required for the NAT gateway to function correctly?
medium
A.The NAT gateway is deployed in a private subnet
B.A security group attached to the NAT gateway
✓ C.A route in the public subnet's route table to an internet gateway
D.A network ACL attached to the NAT gateway's subnet that allows inbound traffic from the internet
✓ E.An Elastic IP address assigned to the NAT gateway
Why C: Option A is required because the NAT gateway needs an Elastic IP for outbound traffic. Option B is required because the public subnet must have a route to an internet gateway. Option C is not required because security groups are not attached to NAT gateways. Option D is not required because NAT gateways are not assigned to private subnets. Option E is not required because NAT gateways do not use NACLs.
Variation 3. A company has a VPC with two subnets: a public subnet with a NAT Gateway and a private subnet. An EC2 instance in the private subnet needs to download patches from the internet. The instance has a security group that allows all outbound traffic. The private subnet's route table has a default route (0.0.0.0/0) pointing to the NAT Gateway. However, the instance cannot reach the internet. What is the most likely issue?
medium
A.The security group does not allow inbound traffic from the internet.
✓ B.The NAT Gateway is deployed in a private subnet.
C.The network ACL on the private subnet blocks outbound traffic.
D.The instance's DNS resolution is not configured correctly.
Why B: The NAT Gateway must be in a public subnet with an Internet Gateway attached. If the NAT Gateway is in a private subnet, it cannot access the internet. Option A (security group) allows outbound; Option B (NACL) default allows; Option D (DNS) is not the issue.
Variation 4. A company has a VPC with a public subnet and a private subnet. They have a NAT Gateway in the public subnet. They also have an EC2 instance in the private subnet that needs to access the internet. The route table for the private subnet has a default route (0.0.0.0/0) pointing to the NAT Gateway. The security group for the EC2 instance allows outbound HTTPS traffic. The network ACL for the private subnet allows inbound and outbound ephemeral ports. However, the EC2 instance cannot reach the internet. The network engineer checks the NAT Gateway and sees that it has an Elastic IP attached. The engineer also checks the route table for the public subnet and finds no route to the internet. What should the engineer do to fix the issue?
medium
A.Deploy a second NAT Gateway in the private subnet.
✓ B.Add a route in the public subnet route table to 0.0.0.0/0 via the internet gateway.
C.Add a route in the private subnet route table to 0.0.0.0/0 via the internet gateway.
D.Attach a second Elastic IP to the NAT Gateway.
Why B: Option C is correct. The public subnet needs a route to an internet gateway for the NAT Gateway to forward traffic to the internet. Option A is wrong because the private subnet route is already correct. Option B is wrong because a second NAT Gateway is not needed. Option D is wrong because the NAT Gateway already has an EIP.
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