The answer is a missing outbound rule for ephemeral ports in the private subnet’s network ACL. This is correct because network ACLs are stateless, meaning they evaluate inbound and outbound traffic independently; even if an inbound rule permits database traffic on port 3306, the outbound rule must explicitly allow the return traffic from the database to the web servers on high-numbered ephemeral ports (typically 1024–65535). Without this rule, the database’s response packets are dropped, causing the intermittent and slow connections. On the AWS Certified SysOps Administrator Associate SOA-02 exam, this scenario tests your understanding of the fundamental stateless versus stateful distinction—a common trap is assuming NACLs behave like security groups, which are stateful and automatically allow return traffic. Remember the mnemonic: “NACLs need two rules—in and out—for every flow; SGs handle the return for you.”
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 company runs a multi-tier web application in a VPC with public and private subnets. The web servers (EC2 instances) are in public subnets, and the database servers (RDS MySQL) are in private subnets. The web servers need to connect to the database servers on port 3306. The security group for the RDS instances (sg-db) has an inbound rule allowing TCP port 3306 from the security group of the web servers (sg-web). The web servers can connect to the database, but the connection is intermittent and slow. The SysOps administrator checks the network ACLs and finds that both the public and private subnet network ACLs have default allow all entries. What is the most likely cause of the issue?
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 private subnet does not have an outbound rule to allow return traffic on ephemeral ports
Option D is correct because the network ACL for the private subnet has no outbound rule to allow return traffic (ephemeral ports) from the database back to the web servers. Network ACLs are stateless, so despite the inbound rule allowing traffic from the web servers, the outbound rule must allow return traffic. Option A is wrong because the security group rule is correct. Option B is wrong because RDS is not a web server; it doesn't need a public subnet. Option C is wrong because NACLs are stateless, not stateful.
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 RDS instances are in a private subnet and cannot receive traffic from public subnets
Why it's wrong here
RDS can receive traffic from public subnets if routing and security allow.
✗
The security group for the RDS instances is missing an inbound rule for port 3306 from the web servers' IP range
Why it's wrong here
The rule using sg-web is correct and more secure.
✓
The network ACL for the private subnet does not have an outbound rule to allow return traffic on ephemeral ports
Why this is correct
Without an outbound rule for ephemeral ports (1024-65535), the DB cannot send response packets back to the web servers.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
The network ACLs are stateful and require an outbound rule for ephemeral ports
Why it's wrong here
NACLs are stateless, so outbound rules are needed for return 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 media company stores terabytes of video archives that are accessed once a year for audit purposes. Moving these objects to a cold storage tier (Azure Archive, S3 Glacier, or Google Nearline) costs a fraction of hot storage. Questions like this test whether you understand storage tiers, access frequency tradeoffs, and retrieval latency requirements.
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 for the private subnet does not have an outbound rule to allow return traffic on ephemeral ports — Option D is correct because the network ACL for the private subnet has no outbound rule to allow return traffic (ephemeral ports) from the database back to the web servers. Network ACLs are stateless, so despite the inbound rule allowing traffic from the web servers, the outbound rule must allow return traffic. Option A is wrong because the security group rule is correct. Option B is wrong because RDS is not a web server; it doesn't need a public subnet. Option C is wrong because NACLs are stateless, not stateful.
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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