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Networking and Content Delivery →mediumMultiple ChoiceObjective-mapped

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.

A
The RDS instances are in a private subnet and cannot receive traffic from public subnets
Why wrong: RDS can receive traffic from public subnets if routing and security allow.
B
The security group for the RDS instances is missing an inbound rule for port 3306 from the web servers' IP range
Why wrong: The rule using sg-web is correct and more secure.
C
The network ACL for the private subnet does not have an outbound rule to allow return traffic on ephemeral ports
Without an outbound rule for ephemeral ports (1024-65535), the DB cannot send response packets back to the web servers.
D
The network ACLs are stateful and require an outbound rule for ephemeral ports
Why wrong: Network ACLs are stateless, meaning they track nothing about the traffic. Therefore, an outbound rule for ephemeral ports is required for return traffic. The statement in the option is incorrect because NACLs are actually stateless, not stateful.

Full breakdown with real-world context →

Answer choices

Why each option matters

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

The correct answer is C. Network ACLs are stateless, meaning they evaluate inbound and outbound traffic separately. The security group on the RDS instances allows inbound traffic on port 3306 from the web servers' security group, so requests from the web servers reach the database. However, the response traffic from the database back to the web servers uses ephemeral ports. If the private subnet's NACL does not have an outbound rule allowing this return traffic, the responses are dropped, causing intermittent and slow connections. Option A is incorrect because RDS in a private subnet can receive traffic from public subnets if allowed by security groups and NACLs. Option B is incorrect because the security group rule is already configured correctly using security group IDs. Option D is incorrect 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.
Related concept
CIDR notation defines the prefix length.
✗
The network ACLs are stateful and require an outbound rule for ephemeral ports
Why it's wrong here
Network ACLs are stateless, meaning they track nothing about the traffic. Therefore, an outbound rule for ephemeral ports is required for return traffic. The statement in the option is incorrect because NACLs are actually stateless, not stateful.

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.

Visual reference

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.

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FAQ

Questions learners often ask

What does this SOA-C02 question test?

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 — The correct answer is C. Network ACLs are stateless, meaning they evaluate inbound and outbound traffic separately. The security group on the RDS instances allows inbound traffic on port 3306 from the web servers' security group, so requests from the web servers reach the database. However, the response traffic from the database back to the web servers uses ephemeral ports. If the private subnet's NACL does not have an outbound rule allowing this return traffic, the responses are dropped, causing intermittent and slow connections. Option A is incorrect because RDS in a private subnet can receive traffic from public subnets if allowed by security groups and NACLs. Option B is incorrect because the security group rule is already configured correctly using security group IDs. Option D is incorrect because NACLs are stateless, not stateful.

What should I do if I get this SOA-C02 question wrong?

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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