The answer is that the engineer forgot to add firewall rules to allow traffic from the peered VPC range. Even with an ACTIVE VPC peering connection, Google Cloud’s default firewall rules deny all ingress traffic, meaning no packets from the peer network are permitted unless an explicit allow rule is created. This is a common trap on the Google Professional Cloud Network Engineer exam: candidates often assume that an active peering status implies connectivity, but peering only establishes routing—firewall rules remain a separate, mandatory layer of control. The exam tests your understanding that VPC peering firewall rules default deny, so you must create ingress allow rules specifying the peer’s IP range as the source. A reliable memory tip is “Peering routes, firewalls gates”—peering opens the path, but firewalls must open the gate.
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.
A network engineer is configuring VPC peering between two VPCs in the same project. The peering status is ACTIVE, but instances in one VPC cannot reach instances in the other VPC using internal IPs. The firewall rules are default (ingress deny all). 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.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The engineer forgot to add firewall rules to allow traffic from the peer range.
Default firewall rules deny all ingress, so even with peering, traffic is blocked unless allow rules are added.
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 VPCs use different routing modes.
Why it's wrong here
Routing mode affects route propagation but not basic connectivity; with ACTIVE peering, routes are exchanged.
✗
The IAM permissions for the peering are missing.
Why it's wrong here
IAM permissions are not needed for existing peering connections in the same project.
✗
The VPCs have overlapping subnet CIDR ranges.
Why it's wrong here
Overlapping subnets would cause the peering to be INACTIVE.
✓
The engineer forgot to add firewall rules to allow traffic from the peer range.
Why this is correct
Firewall rules are required to allow ingress traffic from the peered network.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
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.
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.
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: The engineer forgot to add firewall rules to allow traffic from the peer range. — Default firewall rules deny all ingress, so even with peering, traffic is blocked unless allow rules are added.
What should I do if I get this PCNE question wrong?
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.
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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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.
Question Discussion
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