The answer is the network ACL associated with subnet-B denying inbound traffic from subnet-A. This is because network ACLs are stateless, meaning they require explicit rules for both inbound and outbound traffic, including ephemeral ports for return traffic; if the inbound rule on subnet-B’s NACL does not permit traffic from subnet-A’s CIDR, the packets are silently dropped even though the security groups allow the communication. On the AWS Certified SysOps Administrator Associate SOA-C02 exam, this question tests your understanding of the critical difference between stateful security groups and stateless network ACLs—a common trap is assuming security group rules alone are sufficient, but NACLs act as a subnet-level firewall that must be configured for both directions. Remember that security groups automatically allow return traffic, but NACLs do not, so always check inbound and outbound rules on both subnets. A helpful mnemonic is “NACLs are Not Automatic—Check Both Sides.”
SOA-C02 Networking and Content Delivery Practice Question
This SOA-C02 practice question tests your understanding of networking and content delivery. Match the stated requirement to the specific cloud service, access model, or configuration option — many options are valid in isolation but not for this scenario. 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 a CIDR of 10.0.0.0/16. They have two subnets: subnet-A (10.0.1.0/24) and subnet-B (10.0.2.0/24). An EC2 instance in subnet-A needs to send traffic to an EC2 instance in subnet-B. Both instances are in the same VPC and have appropriate security group rules. However, traffic is not reaching the destination. 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 associated with subnet-B is denying inbound traffic from subnet-A.
Option C is correct because network ACLs are stateless and must allow both inbound and outbound traffic for ephemeral ports. If the NACL on subnet-B does not allow inbound traffic from subnet-A, the traffic will be blocked. Option A is wrong because VPC peering is not needed; both subnets are in the same VPC. Option B is wrong because the route table already has a local route for the VPC CIDR, so no additional routes are needed. Option D is wrong because security groups are stateful; if outbound is allowed, return traffic is automatically allowed.
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 subnet-A does not have a route for subnet-B's CIDR.
Why it's wrong here
The default local route in the VPC's main route table covers all subnets within the VPC CIDR.
✓
The network ACL associated with subnet-B is denying inbound traffic from subnet-A.
Why this is correct
Network ACLs are stateless; if the inbound rule does not allow traffic from subnet-A, it will be blocked.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
The security group on the destination instance does not allow inbound traffic from the source instance.
Why it's wrong here
The question states appropriate security group rules, so this is not the issue.
✗
The VPC peering connection is not established between the two subnets.
Why it's wrong here
Subnets within the same VPC do not need VPC peering; they communicate via the local 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.
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 network ACL associated with subnet-B is denying inbound traffic from subnet-A. — Option C is correct because network ACLs are stateless and must allow both inbound and outbound traffic for ephemeral ports. If the NACL on subnet-B does not allow inbound traffic from subnet-A, the traffic will be blocked. Option A is wrong because VPC peering is not needed; both subnets are in the same VPC. Option B is wrong because the route table already has a local route for the VPC CIDR, so no additional routes are needed. Option D is wrong because security groups are stateful; if outbound is allowed, return traffic is automatically allowed.
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.
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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