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Question 1,116 of 1,546

Networking and Content Delivery →hardMultiple 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 has a multi-tier application with a web tier, application tier, and database tier. All tiers are in the same VPC. The web tier is in public subnets, application tier in private subnets, and database tier in private subnets. The security groups are configured as follows: Web SG allows HTTP/HTTPS from 0.0.0.0/0; App SG allows HTTP from Web SG; DB SG allows MySQL from App SG. The application tier instances cannot connect to the database tier. 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.

A
The VPC CIDR blocks overlap and cause routing issues
Why wrong: One VPC has a single CIDR.
B
The route tables in the private subnets do not have a route to the database subnets
Why wrong: Within a VPC, all subnets can communicate by default.
C
The network ACLs on the database subnets are blocking inbound MySQL traffic
NACLs are stateless; they must allow both directions.
D
The security group for the database tier does not allow return traffic from the application tier
Why wrong: Security groups are stateful; return traffic is automatically allowed.

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 ACLs on the database subnets are blocking inbound MySQL traffic

The application instances cannot connect to the database tier despite the security group rules being correctly configured (DB SG allows MySQL from App SG). Since security groups are stateful and allow return traffic automatically, the issue must be with the network ACLs (NACLs) on the database subnets, which are stateless and require explicit inbound and outbound rules. By default, custom NACLs deny all traffic, so unless they allow MySQL (port 3306) inbound and ephemeral ports outbound, the connection is blocked. Option A is incorrect because overlapping CIDRs are not mentioned and routing within the same VPC is unaffected. Option B is incorrect because route tables in private subnets automatically include a local route to other subnets in the VPC. Option D is incorrect because security groups are stateful and allow return traffic for established connections.

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 VPC CIDR blocks overlap and cause routing issues
Why it's wrong here
One VPC has a single CIDR.
✗
The route tables in the private subnets do not have a route to the database subnets
Why it's wrong here
Within a VPC, all subnets can communicate by default.
✓
The network ACLs on the database subnets are blocking inbound MySQL traffic
Why this is correct
NACLs are stateless; they must allow both directions.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
CIDR notation defines the prefix length.
✗
The security group for the database tier does not allow return traffic from the application tier
Why it's wrong here
Security groups are stateful; return traffic is automatically allowed.

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 ACLs on the database subnets are blocking inbound MySQL traffic — The application instances cannot connect to the database tier despite the security group rules being correctly configured (DB SG allows MySQL from App SG). Since security groups are stateful and allow return traffic automatically, the issue must be with the network ACLs (NACLs) on the database subnets, which are stateless and require explicit inbound and outbound rules. By default, custom NACLs deny all traffic, so unless they allow MySQL (port 3306) inbound and ephemeral ports outbound, the connection is blocked. Option A is incorrect because overlapping CIDRs are not mentioned and routing within the same VPC is unaffected. Option B is incorrect because route tables in private subnets automatically include a local route to other subnets in the VPC. Option D is incorrect because security groups are stateful and allow return traffic for established connections.

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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