- A
Change the priority 300 rule to allow from 0.0.0.0/0 to instances with tag 'health-checked' on TCP port 80.
Why wrong: This would allow all internet traffic to health-checked instances, which is not the intended fix and could expose health check instances incorrectly.
- B
Add a new ingress firewall rule with priority 150 that allows traffic from 130.211.0.0/22 and 35.191.0.0/16 to instances with tag 'web-backend' on TCP port 80.
Why wrong: Creating a new rule works but is not the most efficient because it adds an extra rule. The existing priority 200 rule can be modified to target the correct tag.
- C
Delete the deny-all rule (priority 100) because it is blocking health checks.
Why wrong: Removing the deny-all rule would expose the entire VPC to unrestricted ingress, violating security requirements.
- D
Modify the existing priority 200 rule to set target tags to 'web-backend' instead of 'health-checked'.
This directly fixes the issue by allowing health checks to the MIG instances without creating an additional rule, maintaining the security posture.
Quick Answer
The answer is to modify the existing priority 200 rule’s target tags from 'health-checked' to 'web-backend'. This is correct because Google Cloud health check probes originate from the specific source ranges 130.211.0.0/22 and 35.191.0.0/16, and the priority 200 rule already permits those ranges on TCP port 80—but only to instances tagged 'health-checked'. Since your MIG instances are tagged 'web-backend', the probes are blocked by the higher-priority deny-all rule (priority 100) because the allow rule does not apply to them. By simply changing the target tag on the existing rule, you allow health check traffic to reach the correct instances without creating a new rule or reducing security, as the deny-all still blocks all other traffic. On the Google Professional Cloud Network Engineer exam, this scenario tests your understanding of firewall rule evaluation order and the importance of target tags for health check access to MIG. A common trap is adding a new lower-priority rule, which would never be evaluated due to the implicit deny. Memory tip: “Match the tag, not the rule—health checks need the same tag as your backend.”
PCNE Implementing network security Practice Question
This PCNE practice question tests your understanding of implementing network security. 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.
You are a cloud network engineer for a company that runs a web application on Compute Engine instances in a managed instance group (MIG) behind an external HTTP(S) load balancer. The backend instances are in a subnet with CIDR 10.0.2.0/24 and are tagged 'web-backend'. The health checks are configured to use TCP port 80. Recently, the security team added new firewall rules to restrict traffic, and now the health checks are failing. The current firewall rules (in order of priority) are:
1. Priority 100: Deny ingress from 0.0.0.0/0 to all instances (deny-all). 2. Priority 200: Allow ingress from 130.211.0.0/22 and 35.191.0.0/16 to instances with tag 'health-checked' on TCP port 80. 3. Priority 300: Allow ingress from 0.0.0.0/0 to instances with tag 'web-backend' on TCP port 80.
The MIG instances are tagged 'web-backend' but not 'health-checked'. The health checks are failing. What is the most efficient course of action to fix the health checks while maintaining security?
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
Modify the existing priority 200 rule to set target tags to 'web-backend' instead of 'health-checked'.
The health check probes originate from Google's health checker IP ranges (130.211.0.0/22 and 35.191.0.0/16) and must reach the instances. The current priority 200 rule allows these ranges only to instances tagged 'health-checked', but the MIG instances are tagged 'web-backend'. By modifying the priority 200 rule to target 'web-backend' instead, you permit health check traffic to the correct instances without creating a new rule or lowering security, as the deny-all rule (priority 100) remains in place and the more specific allow rule (priority 200) will match first.
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.
- ✗
Change the priority 300 rule to allow from 0.0.0.0/0 to instances with tag 'health-checked' on TCP port 80.
Why it's wrong here
This would allow all internet traffic to health-checked instances, which is not the intended fix and could expose health check instances incorrectly.
- ✗
Add a new ingress firewall rule with priority 150 that allows traffic from 130.211.0.0/22 and 35.191.0.0/16 to instances with tag 'web-backend' on TCP port 80.
Why it's wrong here
Creating a new rule works but is not the most efficient because it adds an extra rule. The existing priority 200 rule can be modified to target the correct tag.
- ✗
Delete the deny-all rule (priority 100) because it is blocking health checks.
Why it's wrong here
Removing the deny-all rule would expose the entire VPC to unrestricted ingress, violating security requirements.
- ✓
Modify the existing priority 200 rule to set target tags to 'web-backend' instead of 'health-checked'.
Why this is correct
This directly fixes the issue by allowing health checks to the MIG instances without creating an additional rule, maintaining the security posture.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Google Cloud often tests the misconception that you must create a new firewall rule or delete the deny-all rule, rather than simply modifying the target tags of an existing allow rule that already has the correct source IP ranges and protocol.
Detailed technical explanation
How to think about this question
Google Cloud's HTTP(S) load balancer health checks use the Google Cloud health checker IP ranges (130.211.0.0/22 and 35.191.0.0/16) as source addresses. Firewall rules are evaluated in order of priority, and the first matching rule (allow or deny) is applied; the deny-all rule at priority 100 will block all traffic unless a higher-priority (lower number) allow rule matches. By modifying the target tags of the existing priority 200 rule to 'web-backend', you ensure that health check probes are allowed before the deny-all rule is evaluated, while still blocking other traffic from those IP ranges to non-targeted instances.
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
An e-commerce site experiences heavy traffic on Black Friday and near-zero traffic during off-peak weeks. Rather than provisioning permanent large VMs, the team uses auto-scaling groups that add capacity automatically under load and reduce it overnight. Questions like this test whether you understand elasticity, availability zones, and cloud compute scaling patterns.
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 network security — This question tests Implementing network security — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Modify the existing priority 200 rule to set target tags to 'web-backend' instead of 'health-checked'. — The health check probes originate from Google's health checker IP ranges (130.211.0.0/22 and 35.191.0.0/16) and must reach the instances. The current priority 200 rule allows these ranges only to instances tagged 'health-checked', but the MIG instances are tagged 'web-backend'. By modifying the priority 200 rule to target 'web-backend' instead, you permit health check traffic to the correct instances without creating a new rule or lowering security, as the deny-all rule (priority 100) remains in place and the more specific allow rule (priority 200) will match first.
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.
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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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