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Question 95 of 1,000

Implementing a Virtual Private Cloud →mediumMultiple ChoiceObjective-mapped

How to Restrict Access to a Global Application Load Balancer? | Google PCNE

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.

A company has a multi-region VPC with subnets in us-central1 (10.0.0.0/24) and europe-west1 (10.0.1.0/24). They have deployed a global Application Load Balancer (ALB) with backend services in both regions. The backends are instance groups with instances in each subnet. The ALB uses internal IP addresses from a subnet in each region for the backend services. The company wants to restrict access to the ALB so that only traffic from a specific list of external IP addresses (e.g., corporate VPN) can reach the load balancer. They have created a firewall rule allowing ingress from those IP addresses to the ALB's forwarding rule IP (which is a global IP). However, external traffic from allowed IPs is still being blocked. What is the most likely reason?

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 firewall rule should be created with priority lower than 1000 to override default deny.
Why wrong: Default deny egress is not blocking this; the issue is that firewall doesn't apply to global LB frontend.
B
VPC firewall rules do not apply to global load balancer's frontend; use Cloud Armor with IP whitelist.
Global ALB traffic bypasses VPC firewall; Cloud Armor provides access control at the edge.
C
The firewall rule must allow ingress from the allowed IPs to the backend instance's internal IPs.
Why wrong: Traffic from the ALB to backend comes from Google's internal IPs, not the client IPs.
D
The firewall rule must be applied to the backend instances' tags, not the forwarding rule IP.
Why wrong: Backend instances are not directly exposed; the ALB fronts them.

Full breakdown with real-world context →

Quick Answer

The answer is to use Cloud Armor with an IP whitelist, not VPC firewall rules, because VPC firewall rules do not apply to the frontend of a global Application Load Balancer. The global ALB uses an anycast IP that is processed by Google’s frontend infrastructure before traffic ever reaches your VPC network, meaning firewall rules at the subnet or instance level cannot inspect or filter that incoming traffic. This concept is frequently tested on the Google Professional Cloud Network Engineer exam as a classic trap: candidates instinctively apply VPC firewall rules to restrict access, but those rules only govern traffic within the VPC or to backend instances, not the load balancer’s global frontend. Cloud Armor, by contrast, integrates directly with the global ALB to enforce IP-based access controls at the edge. A useful memory tip is “Firewalls for backends, Armor for frontends”—if you need to block or allow external IPs at the load balancer level, always reach for Cloud Armor.

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

✓

VPC firewall rules do not apply to global load balancer's frontend; use Cloud Armor with IP whitelist.

The Global Application Load Balancer (external) uses a global anycast IP that is part of Google's frontend infrastructure, not a VPC resource. VPC firewall rules apply only to traffic entering or leaving VPC network interfaces, such as VM instances or internal load balancers. Traffic to the global ALB's frontend IP is handled before it reaches the VPC, so VPC firewall rules cannot filter it. To restrict access based on source IP addresses, you must use Cloud Armor security policies attached to the load balancer. Therefore, option B is correct. Option A is incorrect because priority is not the issue; firewall rules simply do not apply to the global frontend. Option C and D are incorrect because they refer to backend instances, but the problem is at the frontend. Option B is the correct solution: use Cloud Armor with an IP whitelist.

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 firewall rule should be created with priority lower than 1000 to override default deny.
Why it's wrong here
Default deny egress is not blocking this; the issue is that firewall doesn't apply to global LB frontend.
✓
VPC firewall rules do not apply to global load balancer's frontend; use Cloud Armor with IP whitelist.
Why this is correct
Global ALB traffic bypasses VPC firewall; Cloud Armor provides access control at the edge.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
CIDR notation defines the prefix length.
✗
The firewall rule must allow ingress from the allowed IPs to the backend instance's internal IPs.
Why it's wrong here
Traffic from the ALB to backend comes from Google's internal IPs, not the client IPs.
✗
The firewall rule must be applied to the backend instances' tags, not the forwarding rule IP.
Why it's wrong here
Backend instances are not directly exposed; the ALB fronts them.

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 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.

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 PCNE subnetting questions on CIDR, address ranges, and subnet selection.

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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 — CIDR notation defines the prefix length..

What is the correct answer to this question?

The correct answer is: VPC firewall rules do not apply to global load balancer's frontend; use Cloud Armor with IP whitelist. — The Global Application Load Balancer (external) uses a global anycast IP that is part of Google's frontend infrastructure, not a VPC resource. VPC firewall rules apply only to traffic entering or leaving VPC network interfaces, such as VM instances or internal load balancers. Traffic to the global ALB's frontend IP is handled before it reaches the VPC, so VPC firewall rules cannot filter it. To restrict access based on source IP addresses, you must use Cloud Armor security policies attached to the load balancer. Therefore, option B is correct. Option A is incorrect because priority is not the issue; firewall rules simply do not apply to the global frontend. Option C and D are incorrect because they refer to backend instances, but the problem is at the frontend. Option B is the correct solution: use Cloud Armor with an IP whitelist.

What should I do if I get this PCNE 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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