The answer is the instance's operating system firewall. This is correct because security groups and network ACLs operate at the AWS network layer, but once traffic reaches the EC2 instance itself, the OS-level firewall—such as iptables on Linux or Windows Firewall—can independently filter or drop packets, overriding any permissive cloud-level rules. On the AWS Certified SysOps Administrator Associate SOA-C02 exam, this scenario tests your understanding of the layered security model: AWS controls (security groups and NACLs) govern traffic to the instance, while the OS firewall governs traffic inside the instance. A common trap is assuming that permissive security groups and NACLs guarantee traffic flow, but the OS firewall is a separate, often overlooked gatekeeper. Remember the memory tip: "AWS opens the door, but the OS decides who walks in."
SOA-C02 Networking and Content Delivery Practice Question
This SOA-C02 practice question tests your understanding of networking and content delivery. 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 SysOps administrator notices that traffic to an Amazon EC2 instance is being blocked even though the security group allows all inbound traffic. The subnet's network ACL allows all inbound and outbound traffic. What could be the issue?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The instance's operating system firewall is blocking the traffic.
Option C is correct because the operating system's firewall can block traffic at the instance level. Option A is wrong because NACLs are permissive. Option B is wrong because route tables do not block traffic. Option D is wrong because VPC Flow Logs only log traffic, they do not block it.
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 instance's operating system firewall is blocking the traffic.
Why this is correct
OS-level firewalls (e.g., iptables, Windows Firewall) can block traffic even if AWS firewalls allow it.
The network ACL is not associated with the subnet correctly.
Why it's wrong here
Since the NACL allows all traffic, association is not the issue.
✗
VPC Flow Logs are misconfigured.
Why it's wrong here
Flow logs do not affect traffic; they only capture metadata.
✗
The route table does not have a default route to an internet gateway.
Why it's wrong here
Route tables do not block inbound traffic; they route outbound traffic.
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 SOA-C02 subnetting questions on CIDR, address ranges, and subnet selection.
Networking and Content Delivery — This question tests Networking and Content Delivery — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: The instance's operating system firewall is blocking the traffic. — Option C is correct because the operating system's firewall can block traffic at the instance level. Option A is wrong because NACLs are permissive. Option B is wrong because route tables do not block traffic. Option D is wrong because VPC Flow Logs only log traffic, they do not block it.
What should I do if I get this SOA-C02 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 SOA-C02 subnetting questions on CIDR, address ranges, and subnet selection.
What is the key concept behind this question?
CIDR notation defines the prefix length.
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Question Discussion
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