- A
Configure a VPC firewall rule to deny all ingress traffic to the ILB's forwarding rule IP address, and then create a higher-priority allow rule for the VPC and on-premises ranges
Why wrong: Firewall rules apply to instances, not to forwarding rules directly. ILB traffic passes through firewall rules applied to the backend instances, but the ILB itself is not subject to firewall rules.
- B
Configure the backend service's firewall rules to only allow traffic from the ILB's health check ranges and from the VPC and on-premises source ranges
Correct: Firewall rules on the backend instances allow traffic from the ILB's health check probes and from permitted source ranges, ensuring only desired traffic reaches the application.
- C
Use GKE Network Policy to restrict ingress traffic to the application pods from the VPC and on-premises ranges
Why wrong: GKE Network Policy controls pod-to-pod traffic but does not restrict traffic entering the cluster through the ILB, which arrives at the node and is then routed to the pod.
- D
Use Cloud Armor to create a security policy that allows traffic only from the VPC and on-premises ranges, and attach it to the ILB
Why wrong: Cloud Armor is not supported for Internal Load Balancers; it only works with external load balancers.
Quick Answer
The correct approach is to configure firewall rules directly on the backend service (instance group or NEG) to restrict access to the GKE Internal Load Balancer, allowing only traffic from the ILB’s health check probe ranges—130.211.0.0/22 and 35.191.0.0/16 for GKE—and your permitted source ranges, such as the VPC and on-premises network 10.0.0.0/8. This is the most secure method because it ensures backend instances reject any traffic that hasn’t passed through the ILB, preventing direct access from unauthorized sources and maintaining the principle of least privilege. On the Google Professional Cloud Security Engineer exam, this scenario tests your understanding of defense-in-depth for internal load balancing, where a common trap is applying firewall rules only at the VPC level or on the ILB’s forwarding rule, which still allows packets to reach the backend. A reliable memory tip is “backend first, probes plus sources”—always lock down the backend service with both health check and allowed source ranges to enforce that only ILB-mediated traffic gets through.
PCSE Configuring network security Practice Question
This PCSE practice question tests your understanding of configuring network security. Match the stated requirement to the specific cloud service, access model, or configuration option — many options are valid in isolation but not for this scenario. 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 is deploying a web application on Google Kubernetes Engine (GKE) with an Internal Load Balancer (ILB) as the ingress. The application must only be accessible from within the same VPC and from an on-premises network connected via Cloud VPN. The on-premises network uses IP range 10.0.0.0/8. You have already created the ILB with a backend service. What is the most secure way to restrict access to the ILB?
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
Configure the backend service's firewall rules to only allow traffic from the ILB's health check ranges and from the VPC and on-premises source ranges
Option B is correct because the most secure way to restrict access to an Internal Load Balancer (ILB) is to apply firewall rules directly on the backend service (the instance group or NEG). This ensures that only traffic from the ILB's health check probe ranges (130.211.0.0/22 and 35.191.0.0/16 for GKE) and the allowed source ranges (VPC and on-premises) can reach the backend instances. This approach prevents any traffic from reaching the backend except through the ILB, which is the intended secure design for an internal-only application.
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.
- ✗
Configure a VPC firewall rule to deny all ingress traffic to the ILB's forwarding rule IP address, and then create a higher-priority allow rule for the VPC and on-premises ranges
Why it's wrong here
Firewall rules apply to instances, not to forwarding rules directly. ILB traffic passes through firewall rules applied to the backend instances, but the ILB itself is not subject to firewall rules.
- ✓
Configure the backend service's firewall rules to only allow traffic from the ILB's health check ranges and from the VPC and on-premises source ranges
Why this is correct
Correct: Firewall rules on the backend instances allow traffic from the ILB's health check probes and from permitted source ranges, ensuring only desired traffic reaches the application.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
Use GKE Network Policy to restrict ingress traffic to the application pods from the VPC and on-premises ranges
Why it's wrong here
GKE Network Policy controls pod-to-pod traffic but does not restrict traffic entering the cluster through the ILB, which arrives at the node and is then routed to the pod.
- ✗
Use Cloud Armor to create a security policy that allows traffic only from the VPC and on-premises ranges, and attach it to the ILB
Why it's wrong here
Cloud Armor is not supported for Internal Load Balancers; it only works with external load balancers.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Google Cloud often tests the misconception that VPC firewall rules can be applied to a load balancer's forwarding rule IP address, but in reality, firewall rules only apply to VM instances or GKE nodes, not to forwarding rules or load balancer IPs.
Detailed technical explanation
How to think about this question
When using an ILB with a backend service in GKE, the backend service's firewall rules are evaluated at the instance level, not at the load balancer. The ILB's health check probes must be explicitly allowed (source ranges 130.211.0.0/22 and 35.191.0.0/16) to ensure the backend instances are considered healthy. Without these rules, the ILB will mark the backend as unhealthy and stop forwarding traffic. In a real-world scenario, failing to allow health check ranges is a common misconfiguration that causes the application to become unreachable even if source IP rules are correct.
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 company's IT admin needs to give a contractor read-only access to production logs without sharing account credentials. Using role-based access control (RBAC) and temporary scoped permissions — not a permanent shared password — is the correct pattern. Questions like this test whether you can apply least-privilege access across cloud identity services.
What to study next
Got this wrong? Here's your next step.
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FAQ
Questions learners often ask
What does this PCSE question test?
Configuring network security — This question tests Configuring network security — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Configure the backend service's firewall rules to only allow traffic from the ILB's health check ranges and from the VPC and on-premises source ranges — Option B is correct because the most secure way to restrict access to an Internal Load Balancer (ILB) is to apply firewall rules directly on the backend service (the instance group or NEG). This ensures that only traffic from the ILB's health check probe ranges (130.211.0.0/22 and 35.191.0.0/16 for GKE) and the allowed source ranges (VPC and on-premises) can reach the backend instances. This approach prevents any traffic from reaching the backend except through the ILB, which is the intended secure design for an internal-only application.
What should I do if I get this PCSE 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 PCSE 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 PCSE exam.
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