The answer is a private IP range overlap between the Cloud SQL instance and the application's VPC. This is correct because VPC peering requires strictly non-overlapping CIDR blocks to establish functional routing tables; when the Cloud SQL private IP range conflicts with the application's VPC, the peering connection cannot route traffic correctly, causing connectivity failures while public IP remains unaffected. On the Google Professional Cloud Database Engineer exam, this scenario tests your understanding of VPC peering constraints and Cloud SQL networking, often appearing as a trick where candidates mistakenly blame firewall rules or DNS. The common trap is assuming public IP failure first, but since public IP works, the issue is isolated to private IP routing. Remember the memory tip: "Peering needs non-overlapping, or your private IP is disappearing."
PCDE Plan and manage database infrastructure Practice Question
This PCDE practice question tests your understanding of plan and manage database infrastructure. 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 developer reports that their application cannot connect to a Cloud SQL instance using private IP, but public IP works. The Cloud SQL instance is in VPC peering with the application's VPC. The application is in the same region. 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.
Why wrong: If peering were missing, public IP would likely also fail if the application is in a different network; but public works, so peering is established.
B
The private IP range of the Cloud SQL instance conflicts with the application's VPC.
IP overlap in peered VPCs causes routing issues, preventing private IP connectivity while public IP remains unaffected.
C
The Cloud SQL proxy is not running.
Why wrong: The Cloud SQL Proxy is used for IAM-based connections but is not required for direct private IP connectivity.
D
The Cloud SQL instance has 'require SSL' enabled.
Why wrong: SSL requirement would affect both private and public connections equally, not just private.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The private IP range of the Cloud SQL instance conflicts with the application's VPC.
Option B is correct because when a Cloud SQL instance is configured with a private IP address that overlaps with the application's VPC CIDR range, the VPC peering connection cannot route traffic correctly. This is due to the fact that VPC peering requires non-overlapping IP ranges to establish proper routing tables; overlapping ranges cause route conflicts and connectivity failures. Since public IP works, the issue is isolated to private IP routing, making IP range conflict the most likely cause.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
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 peering connection is not established.
Why it's wrong here
If peering were missing, public IP would likely also fail if the application is in a different network; but public works, so peering is established.
✓
The private IP range of the Cloud SQL instance conflicts with the application's VPC.
Why this is correct
IP overlap in peered VPCs causes routing issues, preventing private IP connectivity while public IP remains unaffected.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
The Cloud SQL proxy is not running.
Why it's wrong here
The Cloud SQL Proxy is used for IAM-based connections but is not required for direct private IP connectivity.
✗
The Cloud SQL instance has 'require SSL' enabled.
Why it's wrong here
SSL requirement would affect both private and public connections equally, not just private.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Google Cloud often tests the misconception that VPC peering automatically handles overlapping IP ranges, when in fact overlapping ranges cause routing failures that prevent private IP connectivity even if the peering connection itself is established.
Detailed technical explanation
How to think about this question
VPC peering uses routes to direct traffic between peered networks; if the Cloud SQL instance's private IP (e.g., 10.0.0.5) falls within the application VPC's CIDR (e.g., 10.0.0.0/24), the route for that IP is local to the application VPC and never forwarded to the peered Cloud SQL VPC. This is governed by the principle of longest prefix match in routing tables, where a more specific local route takes precedence over a peering route. In real-world scenarios, this often occurs when organizations use overlapping RFC 1918 address ranges (e.g., 10.0.0.0/8) across different projects.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
TExam Day Tips
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A healthcare organisation deploys an application with a public-facing web tier and a private database tier. The database subnet has no public IP and only accepts connections from the web tier's security group. Questions like this test whether you can design cloud network isolation using VNets/VPCs, subnets, and security group rules.
What to study next
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Plan and manage database infrastructure — This question tests Plan and manage database infrastructure — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The private IP range of the Cloud SQL instance conflicts with the application's VPC. — Option B is correct because when a Cloud SQL instance is configured with a private IP address that overlaps with the application's VPC CIDR range, the VPC peering connection cannot route traffic correctly. This is due to the fact that VPC peering requires non-overlapping IP ranges to establish proper routing tables; overlapping ranges cause route conflicts and connectivity failures. Since public IP works, the issue is isolated to private IP routing, making IP range conflict the most likely cause.
What should I do if I get this PCDE question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
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?
Read the scenario before looking for a memorised answer.
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