This SCS-C02 practice question tests your understanding of infrastructure security. 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 security engineer is troubleshooting connectivity issues between two EC2 instances in the same VPC but different subnets. Both instances have security groups that allow all traffic from each other's security group. However, traffic is still blocked. 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 one or both subnets is blocking the traffic.
Option C is correct because network ACLs are stateless and must allow both inbound and outbound traffic for the connection. Even if security groups allow traffic, a network ACL can block it. Option A is wrong because route tables handle routing, not filtering. Option B is wrong because VPC flow logs only monitor traffic, not block it. Option D is wrong because the default VPC has a default network ACL that allows all traffic, but custom ACLs may have restrictive rules.
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 instances are in different VPCs.
Why it's wrong here
The scenario states same VPC.
✓
The network ACL for one or both subnets is blocking the traffic.
Why this is correct
Network ACLs are stateless and can block traffic even if security groups allow it.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
The route tables do not have a route between the subnets.
Why it's wrong here
Route tables enable routing, but the main VPC route table typically allows local traffic.
✗
VPC Flow Logs are not enabled.
Why it's wrong here
Flow logs do not affect traffic flow.
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
The scenario states same VPC.
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 SCS-C02 subnetting questions on CIDR, address ranges, and subnet selection.
Infrastructure Security — This question tests Infrastructure Security — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: The network ACL for one or both subnets is blocking the traffic. — Option C is correct because network ACLs are stateless and must allow both inbound and outbound traffic for the connection. Even if security groups allow traffic, a network ACL can block it. Option A is wrong because route tables handle routing, not filtering. Option B is wrong because VPC flow logs only monitor traffic, not block it. Option D is wrong because the default VPC has a default network ACL that allows all traffic, but custom ACLs may have restrictive rules.
What should I do if I get this SCS-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 SCS-C02 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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Question Discussion
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