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Question 37 of 1,705

Network Design →mediumMultiple ChoiceObjective-mapped

ANS-C01 Network Design Practice Question

This ANS-C01 practice question tests your understanding of network design. Examine the command output carefully: the correct answer depends on what the output actually shows, not on general recall alone. 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 deployed a multi-tier web application in a single AWS region. The architecture includes a VPC with public and private subnets across two Availability Zones. The web tier uses an Application Load Balancer (ALB) in the public subnets, and the application tier runs on EC2 instances in the private subnets. The database tier uses an Amazon RDS Multi-AZ deployment in the database subnets. The company is experiencing intermittent connectivity issues between the application tier and the database tier. The application logs show connection timeouts. The network engineer has verified that the security groups and network ACLs are correctly configured. The RDS instance is reachable from the application tier via a telnet test from one specific instance, but not consistently from all instances. What is the most likely cause of the intermittent connectivity?

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 Multi-AZ failover is causing the primary instance to change, and the application is not reconnecting to the new endpoint.
Correct. RDS Multi-AZ failover can cause the primary endpoint IP to change. If the application does not properly handle DNS changes or uses stale connections, it may experience intermittent connectivity after a failover.
B
The network ACLs on the database subnets are blocking ephemeral ports used by the application.
Why wrong: Incorrect. Network ACLs are stateless but if correctly configured, they would not block ephemeral ports intermittently. The issue is not consistent, making this unlikely.
C
The database subnets are in different Availability Zones than the application subnets, and the route tables in the application subnets do not have routes to the database subnet CIDRs.
Why wrong: Incorrect. In a single VPC, the local route automatically enables communication between all subnets. Missing routes to database subnet CIDRs is not possible within the same VPC.
D
The security group for the database is allowing traffic only from the application tier's security group, but the application tier instances are using a different security group.
Why wrong: Incorrect. If the security group for the database allowed traffic only from a different security group than the one used by the application instances, connectivity would consistently fail from all instances, not intermittently.

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 RDS Multi-AZ failover is causing the primary instance to change, and the application is not reconnecting to the new endpoint.

The most likely cause is that the RDS Multi-AZ failover is causing the primary instance to change, and the application is not reconnecting to the new endpoint. In a Multi-AZ deployment, if a failover occurs, the DNS endpoint updates to point to the new primary, but if the application caches the resolved IP address or uses stale connections, it may experience intermittent timeouts. This explains why connectivity works from some instances (those with fresh DNS entries) but not others. Option C is incorrect because within a single VPC, the local route automatically enables routing between all subnets; missing routes would cause consistent failure, not intermittent. Option B is incorrect because network ACLs, if correctly configured, would not cause intermittent issues. Option D is incorrect because a security group misconfiguration would likely cause consistent failure from all instances, not intermittent connectivity.

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 Multi-AZ failover is causing the primary instance to change, and the application is not reconnecting to the new endpoint.
Why this is correct
Correct. RDS Multi-AZ failover can cause the primary endpoint IP to change. If the application does not properly handle DNS changes or uses stale connections, it may experience intermittent connectivity after a failover.
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 on the database subnets are blocking ephemeral ports used by the application.
Why it's wrong here
Incorrect. Network ACLs are stateless but if correctly configured, they would not block ephemeral ports intermittently. The issue is not consistent, making this unlikely.
✗
The database subnets are in different Availability Zones than the application subnets, and the route tables in the application subnets do not have routes to the database subnet CIDRs.
Why it's wrong here
Incorrect. In a single VPC, the local route automatically enables communication between all subnets. Missing routes to database subnet CIDRs is not possible within the same VPC.
✗
The security group for the database is allowing traffic only from the application tier's security group, but the application tier instances are using a different security group.
Why it's wrong here
Incorrect. If the security group for the database allowed traffic only from a different security group than the one used by the application instances, connectivity would consistently fail from all instances, not intermittently.

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 ANS-C01 subnetting questions on CIDR, address ranges, and subnet selection.

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FAQ

Questions learners often ask

What does this ANS-C01 question test?

Network Design — This question tests Network Design — CIDR notation defines the prefix length..

What is the correct answer to this question?

The correct answer is: The RDS Multi-AZ failover is causing the primary instance to change, and the application is not reconnecting to the new endpoint. — The most likely cause is that the RDS Multi-AZ failover is causing the primary instance to change, and the application is not reconnecting to the new endpoint. In a Multi-AZ deployment, if a failover occurs, the DNS endpoint updates to point to the new primary, but if the application caches the resolved IP address or uses stale connections, it may experience intermittent timeouts. This explains why connectivity works from some instances (those with fresh DNS entries) but not others. Option C is incorrect because within a single VPC, the local route automatically enables routing between all subnets; missing routes would cause consistent failure, not intermittent. Option B is incorrect because network ACLs, if correctly configured, would not cause intermittent issues. Option D is incorrect because a security group misconfiguration would likely cause consistent failure from all instances, not intermittent connectivity.

What should I do if I get this ANS-C01 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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