- A
The instance in vpc-b has a firewall rule that denies TCP port 8080 explicitly.
Why wrong: There is no evidence of a deny rule; the symptom suggests the allow rule is not matching.
- B
The subnet in vpc-a is in a different region, so peering does not support cross-region communication.
Why wrong: VPC Network Peering supports cross-region communication, so this is not the issue.
- C
The firewall rule for port 8080 is missing or uses an incorrect target tag or service account.
Since ICMP works but TCP 8080 does not, the specific firewall rule for port 8080 is likely missing or misconfigured.
- D
The firewall rule in vpc-b must allow ingress from the entire CIDR of vpc-a's subnet, not just the peered network.
Why wrong: Using the peered network as source is equivalent to the subnet CIDR; that is not the issue.
Quick Answer
The answer is that the firewall rule for TCP port 8080 is either missing or misconfigured with an incorrect target tag or service account. This is correct because VPC peering does not automatically grant traffic flow; it only establishes Layer 3 connectivity, meaning ICMP can succeed while a specific TCP port remains blocked if no explicit ingress firewall rule permits it. In the Google Professional Cloud Network Engineer exam, this scenario tests your understanding that peered networks remain administratively separate—firewall rules must be independently applied in the destination VPC, and a common trap is assuming peering implies full access. The key distinction here is that ICMP works (proving routing and peering are fine), but TCP 8080 fails, isolating the issue to a missing or mis-tagged firewall rule for that port. Memory tip: “Peering opens the door, but firewalls decide which rooms you can enter.”
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 two VPC networks in the same project: 'vpc-a' (us-central1) and 'vpc-b' (us-east1). They are connected via VPC Network Peering. An instance in vpc-a can ping the internal IP of an instance in vpc-b, but cannot reach it on TCP port 8080. The firewall rule in vpc-b allows ingress from the peered network's subnets. 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 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
The firewall rule for port 8080 is missing or uses an incorrect target tag or service account.
The firewall rule in vpc-b allows ingress from the peered network's subnets, but VPC Network Peering does not automatically allow all traffic; firewalls must be configured explicitly. Since ICMP works, the issue is specific to TCP 8080, likely a firewall rule missing or misconfigured for that port. Option D is correct because even if the rule allows from subnets, it may not have the correct port or target tags.
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 instance in vpc-b has a firewall rule that denies TCP port 8080 explicitly.
Why it's wrong here
There is no evidence of a deny rule; the symptom suggests the allow rule is not matching.
- ✗
The subnet in vpc-a is in a different region, so peering does not support cross-region communication.
Why it's wrong here
VPC Network Peering supports cross-region communication, so this is not the issue.
- ✓
The firewall rule for port 8080 is missing or uses an incorrect target tag or service account.
- ✗
The firewall rule in vpc-b must allow ingress from the entire CIDR of vpc-a's subnet, not just the peered network.
Why it's wrong here
Using the peered network as source is equivalent to the subnet CIDR; that is not the issue.
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 healthcare organisation deploys an application with a public-facing web tier and a private database tier. The database subnet has no public IP and only accepts connections from the web tier's security group. Questions like this test whether you can design cloud network isolation using VNets/VPCs, subnets, and security group rules.
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: The firewall rule for port 8080 is missing or uses an incorrect target tag or service account. — The firewall rule in vpc-b allows ingress from the peered network's subnets, but VPC Network Peering does not automatically allow all traffic; firewalls must be configured explicitly. Since ICMP works, the issue is specific to TCP 8080, likely a firewall rule missing or misconfigured for that port. Option D is correct because even if the rule allows from subnets, it may not have the correct port or target tags.
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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Last reviewed: Jun 24, 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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