The answer is that VPC firewall rules are blocking ingress from on-premises, so you must add a rule allowing traffic from 192.168.0.0/16 to the database IP. This is because Cloud VPN and BGP handle routing—ensuring packets know the path—but they do not override VPC firewall policies, which control whether packets are actually permitted to reach the target instance. Even with a correctly advertised subnet via BGP and a permissive OS firewall on the database, the VPC’s implicit deny ingress rule will drop traffic from the on-premises 192.168.0.0/16 range unless an explicit allow rule is created. On the Google Professional Cloud Network Engineer exam, this scenario tests your understanding that Cloud VPN BGP firewall rule connectivity issues often stem from misapplied network security layers, not routing. A common trap is assuming that a successful BGP route exchange or a permissive instance-level firewall guarantees connectivity. Memory tip: “Routes guide, firewalls decide—BGP gets you there, but VPC firewalls let you in.”
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 company has a VPC with a single subnet in us-central1 (10.0.0.0/24). They have a Compute Engine instance running a database that uses an internal IP address 10.0.0.10. They need to ensure that this database instance can be accessed by a legacy on-premises application via a Cloud VPN tunnel. The on-premises network uses 192.168.0.0/16. They have set up a HA VPN gateway with two tunnels and BGP routing. The Cloud Router is configured to advertise the subnet 10.0.0.0/24. On the on-premises side, the router receives the route for 10.0.0.0/24 and has a static route for 10.0.0.0/24 pointing to the VPN tunnel. However, the on-premises application cannot reach the database. The application's server can ping the on-premises gateway, but not the database IP. The database instance's OS firewall allows all traffic from 0.0.0.0/0. 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 VPC firewall rules are blocking ingress from on-premises; add a rule allowing traffic from 192.168.0.0/16 to the database IP.
The issue is likely that the VPC firewall rules are blocking ingress traffic from the on-premises network. Even though the database OS firewall is permissive, the VPC firewall must allow ingress from the on-premises IP range (192.168.0.0/16) to the database's IP. Option B is correct: Create a firewall rule allowing ingress from 192.168.0.0/16 to 10.0.0.10 on the required port.
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 database instance's OS firewall is blocking the traffic despite the setting.
Why it's wrong here
The OS firewall is set to allow all, so it is not the issue.
✓
The VPC firewall rules are blocking ingress from on-premises; add a rule allowing traffic from 192.168.0.0/16 to the database IP.
Why this is correct
By default, VPC firewall denies ingress; an explicit allow rule is needed.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
The Cloud Router is not advertising the specific database IP 10.0.0.10, only the subnet 10.0.0.0/24.
Why it's wrong here
Advertising the subnet is sufficient; the on-premises router will route to the entire subnet.
✗
The BGP session is not establishing properly; check the shared secret and IP addresses.
Why it's wrong here
The on-premises router received the route, so BGP is working.
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.
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 VPC firewall rules are blocking ingress from on-premises; add a rule allowing traffic from 192.168.0.0/16 to the database IP. — The issue is likely that the VPC firewall rules are blocking ingress traffic from the on-premises network. Even though the database OS firewall is permissive, the VPC firewall must allow ingress from the on-premises IP range (192.168.0.0/16) to the database's IP. Option B is correct: Create a firewall rule allowing ingress from 192.168.0.0/16 to 10.0.0.10 on the required port.
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.
About these practice questions
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These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. An engineer is troubleshooting connectivity between an on-premises network and a GCP VPC over a Cloud VPN tunnel with dynamic routing (BGP). The tunnel is established and BGP session is up, but on-premises hosts cannot reach instances in the VPC. What should the engineer check first?
medium
A.The advertised route from the on-premises router is a default route.
B.The MTU size of the VPN tunnel.
C.The Cloud VPN gateway is assigned an external IP address.
✓ D.The firewall rules in the VPC allowing incoming traffic from the on-premises CIDR.
Why D: Option C is correct because even with BGP routes, the VPC firewall rules must permit incoming traffic from the on-premises CIDR. If no appropriate ingress rule exists, traffic will be blocked. Option A is incorrect because MTU might cause packet loss but not complete failure. Option B is incorrect because advertised routes from the on-premises router are being learned (BGP is up). Option D is incorrect because the VPN gateway's external IP is necessary for the tunnel but not the immediate cause of connectivity failure.
Last reviewed: Jun 24, 2026
Question Discussion
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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.
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