- A
Create a custom route from the web servers to the database.
Why wrong: The web servers can ping the database, so routing is working; a custom route is unnecessary.
- B
Check the database instance's OS-level firewall (e.g., iptables) and ensure it allows traffic on port 3306 from the web servers.
The VPC firewall allows traffic, but the OS firewall on the database may be blocking the connection.
- C
Add a network tag 'db' to the database instance and update the firewall rule to target 'db'.
Why wrong: The existing firewall rule already targets the database IP, so a tag is not needed; the instance might still have OS firewall blocking.
- D
Configure the firewall rule to use the database's service account as the target.
Why wrong: Service account targeting is for VMs, but the database's OS firewall is the likely culprit.
Quick Answer
The answer is to check the database instance’s OS-level firewall, such as iptables, because VPC firewall rules and guest OS firewalls operate independently in Google Cloud. Even though your VPC firewall rule correctly allows ingress from 10.0.1.0/24 to the database on TCP 3306, and ping succeeds (which uses ICMP, not TCP), the OS firewall on the database VM may still be blocking port 3306, preventing the application-layer connection. This scenario tests your understanding of the layered security model on the Google Professional Cloud Network Engineer exam, where a common trap is assuming VPC rules alone guarantee connectivity. Remember that cloud firewalls control network traffic at the hypervisor level, while iptables controls it inside the VM’s kernel—both must permit the traffic. A useful memory tip: “VPC rules open the cloud door, but iptables locks the room door.”
PCNE Implementing a Virtual Private Cloud Practice Question
This PCNE practice question tests your understanding of implementing a virtual private cloud. 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.
Your company has a VPC with a single subnet in us-central1 (10.0.1.0/24). You have a managed instance group (MIG) of web servers (10.0.1.2-10.0.1.10) and a standalone database instance (10.0.1.100). The web servers need to communicate with the database on TCP port 3306. You have configured a firewall rule allowing ingress from 10.0.1.0/24 to 10.0.1.100 on tcp:3306. However, the web servers cannot connect to the database. You verified that the database is running and listening on port 3306, and that the web servers can ping the database. What should you do to resolve the issue?
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
Check the database instance's OS-level firewall (e.g., iptables) and ensure it allows traffic on port 3306 from the web servers.
The firewall rule is correctly configured at the VPC level to allow ingress from the web servers' subnet to the database on TCP port 3306. Since the web servers can ping the database, network connectivity exists, but the application-layer connection fails. This indicates the database instance's OS-level firewall (e.g., iptables on Linux) is blocking the port, as cloud VPC firewall rules and guest OS firewalls operate independently.
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.
- ✗
Create a custom route from the web servers to the database.
Why it's wrong here
The web servers can ping the database, so routing is working; a custom route is unnecessary.
- ✓
Check the database instance's OS-level firewall (e.g., iptables) and ensure it allows traffic on port 3306 from the web servers.
Why this is correct
The VPC firewall allows traffic, but the OS firewall on the database may be blocking the connection.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
Add a network tag 'db' to the database instance and update the firewall rule to target 'db'.
Why it's wrong here
The existing firewall rule already targets the database IP, so a tag is not needed; the instance might still have OS firewall blocking.
- ✗
Configure the firewall rule to use the database's service account as the target.
Why it's wrong here
Service account targeting is for VMs, but the database's OS firewall is the likely culprit.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Google Cloud often tests the distinction between cloud-level firewall rules and guest OS firewalls, leading candidates to overcomplicate the solution with network tags or routes when the issue is a simple OS-level block.
Detailed technical explanation
How to think about this question
In Google Cloud, VPC firewall rules are stateful and filter traffic at the hypervisor level before it reaches the instance. However, the guest OS firewall (e.g., iptables, ufw, or Windows Firewall) runs inside the instance and can further restrict traffic. A common scenario is when a default deny policy on the OS blocks non-essential ports, requiring explicit allow rules for database ports like 3306. Tools like `iptables -L -n` or `ss -tlnp` can verify OS-level rules.
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.
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FAQ
Questions learners often ask
What does this PCNE question test?
Implementing a Virtual Private Cloud — This question tests Implementing a Virtual Private Cloud — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Check the database instance's OS-level firewall (e.g., iptables) and ensure it allows traffic on port 3306 from the web servers. — The firewall rule is correctly configured at the VPC level to allow ingress from the web servers' subnet to the database on TCP port 3306. Since the web servers can ping the database, network connectivity exists, but the application-layer connection fails. This indicates the database instance's OS-level firewall (e.g., iptables on Linux) is blocking the port, as cloud VPC firewall rules and guest OS firewalls operate independently.
What should I do if I get this PCNE question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
About these practice questions
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Last reviewed: Jun 30, 2026
This PCNE practice question is part of Courseiva's free Google Cloud certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the PCNE exam.
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