The answer is that VPC Flow Logs do not log intra-subnet traffic because such traffic stays at layer 2 and never traverses the VPC router. When two Compute Engine instances communicate within the same subnet, the packets are forwarded directly at the hypervisor level without passing through the virtual network stack that flow logs monitor. This is a common trap on the Google Professional Cloud Network Engineer exam: candidates often assume that enabling flow logs with a 1.0 sample rate captures all traffic, but the key technical concept is that flow logs only capture traffic that crosses subnet boundaries or reaches external gateways. The exam tests your understanding that VPC Flow Logs operate at the VPC router, not at the host level. A useful memory tip: "Same subnet, same switch—no router, no log."
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 wants to enable VPC Flow Logs for a subnet to troubleshoot connectivity issues. They have enabled flow logs with a sample rate of 1.0 and metadata annotation enabled. After a few hours, they notice that logs are being generated but they are missing flows from a specific application server to a database server in the same subnet. Both servers are Compute Engine instances with internal IPs only. What could be the cause?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
VPC Flow Logs do not capture traffic between instances in the same subnet because it does not traverse the VPC router.
VPC Flow Logs capture only IP traffic that is logged for both ingress and egress. If the application server and database server are in the same subnet, traffic between them is layer 2 (if they are on the same VPC) and may not be captured because flow logs apply to traffic that traverses the virtual network stack. Traffic within the same subnet is typically forwarded directly without going through the VPC router, so flow logs may not capture it. Option A is correct.
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 instances are using internal DNS names, and flow logs only capture traffic by IP address.
Why it's wrong here
Flow logs capture IP traffic regardless of DNS names.
✗
The sample rate of 1.0 means all flows are sampled, but metadata annotation may be causing some flows to be dropped.
Why it's wrong here
Sample rate 1.0 captures all flows, and metadata annotation does not drop flows.
✗
Flow logs must be enabled on the VPC network, not just the subnet.
Why it's wrong here
Flow logs are enabled per subnet; they work when enabled on the subnet.
✓
VPC Flow Logs do not capture traffic between instances in the same subnet because it does not traverse the VPC router.
Why this is correct
Traffic within the same subnet is sent directly at layer 2, bypassing the router where flow logs are captured.
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
An e-commerce site experiences heavy traffic on Black Friday and near-zero traffic during off-peak weeks. Rather than provisioning permanent large VMs, the team uses auto-scaling groups that add capacity automatically under load and reduce it overnight. Questions like this test whether you understand elasticity, availability zones, and cloud compute scaling patterns.
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: VPC Flow Logs do not capture traffic between instances in the same subnet because it does not traverse the VPC router. — VPC Flow Logs capture only IP traffic that is logged for both ingress and egress. If the application server and database server are in the same subnet, traffic between them is layer 2 (if they are on the same VPC) and may not be captured because flow logs apply to traffic that traverses the virtual network stack. Traffic within the same subnet is typically forwarded directly without going through the VPC router, so flow logs may not capture it. Option A is correct.
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.
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.
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