The answer is the network ACL blocking outbound traffic. Even when an EC2 instance in a public subnet has a public IP and a route table with a default route pointing to an internet gateway, the subnet’s network ACL acts as a stateless firewall that must explicitly allow both outbound traffic and the corresponding inbound return traffic on ephemeral ports. If the outbound rule is missing or overly restrictive, packets leave the subnet but the return traffic is dropped, making the internet unreachable. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this scenario tests your understanding of the layered security model: route tables control the path, but NACLs control packet-level permissions. A common trap is assuming a public IP and correct route alone guarantee connectivity, forgetting that NACLs are stateless and require bidirectional rules. Memory tip: “NACL is stateless—it needs a two-way handshake for every flow.”
ANS-C01 Network Management and Operations Practice Question
This ANS-C01 practice question tests your understanding of network management and operations. 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 network engineer is troubleshooting an issue where an EC2 instance in a public subnet cannot reach the internet. The instance has a public IP, and the route table has a default route to an internet 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 is blocking outbound traffic
Even with correct routes, if the subnet's network ACL does not allow outbound traffic, the instance cannot reach the internet. Network ACLs are stateless and must allow both outbound and inbound ephemeral ports.
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 network ACL is blocking outbound traffic
Why this is correct
Network ACLs are stateless and must explicitly allow outbound traffic and inbound return traffic.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
If the internet gateway were not attached, the route to it would be invalid, but the scenario says the route exists.
✗
The security group does not allow outbound HTTP traffic
Why it's wrong here
Security groups are stateful; if outbound traffic is allowed, return traffic is automatically allowed.
✗
The instance does not have a public IP
Why it's wrong here
The scenario states the instance has a public IP.
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
Scenario analysis trap
If the internet gateway were not attached, the route to it would be invalid, but the scenario says the route exists.
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 Management and Operations — This question tests Network Management and Operations — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: The network ACL is blocking outbound traffic — Even with correct routes, if the subnet's network ACL does not allow outbound traffic, the instance cannot reach the internet. Network ACLs are stateless and must allow both outbound and inbound ephemeral ports.
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
Courseiva creates original exam-style practice questions with explanations and wrong-answer analysis. It does not publish real exam questions, exam dumps, or protected exam content. Learn why practice questions differ from exam dumps →
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 network engineer notices that traffic from an EC2 instance in a public subnet to the internet is not working. The instance has a public IP assigned and is in a public subnet with a route to an internet gateway. The security group allows outbound traffic. What should the engineer check next?
medium
A.Ensure the network ACL allows outbound traffic.
✓ B.Verify that the route table for the subnet has a 0.0.0.0/0 route pointing to the internet gateway.
C.Confirm that the internet gateway is attached to the VPC.
D.Check if the instance has a public IP assigned.
Why B: Even if the instance has a public IP, if the subnet's route table does not have a default route (0.0.0.0/0) pointing to the internet gateway, traffic to the internet will fail. This is a common misconfiguration.
Last reviewed: Jun 20, 2026
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