The correct answer is that the network ACLs on the database subnets are blocking inbound MySQL traffic. This is because network ACLs are stateless, meaning they require explicit rules for both inbound and outbound traffic; even if the database security group allows MySQL from the application tier, a restrictive NACL on the database subnet can silently drop the connection requests. On the AWS Certified SysOps Administrator Associate SOA-C02 exam, this scenario tests your understanding of the critical difference between stateful security groups and stateless network ACLs—a common trap is assuming security group rules alone guarantee connectivity, while forgetting that NACLs act as a separate, subnet-level firewall. To remember this, think of NACLs as the “bouncer at the subnet door” who needs to see both the entry and exit passes, whereas security groups are like a “smart doorman” who remembers who came in.
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.
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
Option D is correct because network ACLs are stateless and must allow both inbound and outbound traffic for the connection to work. Even if security groups allow traffic, NACLs can block it. Option A is wrong because security groups are stateful and allow return traffic automatically. Option B is wrong because the VPC CIDR is large enough. Option C is wrong because route tables are not the issue if all are in the same VPC.
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
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.
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 ACLs on the database subnets are blocking inbound MySQL traffic — Option D is correct because network ACLs are stateless and must allow both inbound and outbound traffic for the connection to work. Even if security groups allow traffic, NACLs can block it. Option A is wrong because security groups are stateful and allow return traffic automatically. Option B is wrong because the VPC CIDR is large enough. Option C is wrong because route tables are not the issue if all are in the same VPC.
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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Question Discussion
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