- A
Configure the appliance's guest OS to enable packet forwarding by setting net.ipv4.ip_forward=1.
Why wrong: This is already required by enabling IP forwarding on the instance, but the VPC still needs a route.
- B
Create a firewall rule allowing egress from the appliance to both subnets.
Why wrong: Firewall rules are necessary but not sufficient; routing must be configured.
- C
Attach the appliance to a Cloud Router and configure BGP sessions.
Why wrong: Cloud Router is for dynamic routing with VPNs or Dedicated Interconnect, not for simple subnet-to-subnet forwarding.
- D
Create a route in the VPC with destination subnet B and next hop set to the appliance's internal IP address.
This custom route directs traffic to the appliance for forwarding.
Question 525 of 1,000
PCNE Implementing VPC Instances Practice Question
This PCNE practice question tests your understanding of implementing vpc instances. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. 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 deployed a network appliance (e.g., firewall) as a Compute Engine instance with two NICs: NIC0 for management and NIC1 for data traffic. The appliance must forward traffic from instances in subnet A to subnet B. The engineer has enabled IP forwarding on the appliance. What additional configuration is required on the VPC for the appliance to route traffic between subnets?
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
✓
Create a route in the VPC with destination subnet B and next hop set to the appliance's internal IP address.
For a Compute Engine instance to forward traffic between subnets, it must act as a router. The VPC needs a custom route that sends traffic destined for subnet B to the appliance's IP (or NIC) as the next hop. Without this route, the VPC's default routing will deliver traffic directly between subnets, bypassing the appliance. Also, the appliance's NIC1 must be in the same VPC and the firewall rules must allow the traffic.
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.
- ✗
Configure the appliance's guest OS to enable packet forwarding by setting net.ipv4.ip_forward=1.
Why it's wrong here
This is already required by enabling IP forwarding on the instance, but the VPC still needs a route.
- ✗
Create a firewall rule allowing egress from the appliance to both subnets.
Why it's wrong here
Firewall rules are necessary but not sufficient; routing must be configured.
- ✗
Attach the appliance to a Cloud Router and configure BGP sessions.
Why it's wrong here
Cloud Router is for dynamic routing with VPNs or Dedicated Interconnect, not for simple subnet-to-subnet forwarding.
- ✓
Create a route in the VPC with destination subnet B and next hop set to the appliance's internal IP address.
Why this is correct
This custom route directs traffic to the appliance for forwarding.
Related concept
CIDR notation defines the prefix length.
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
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.
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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Implementing VPC Instances — study guide chapter
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FAQ
Questions learners often ask
What does this PCNE question test?
Implementing VPC Instances — This question tests Implementing VPC Instances — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: Create a route in the VPC with destination subnet B and next hop set to the appliance's internal IP address. — For a Compute Engine instance to forward traffic between subnets, it must act as a router. The VPC needs a custom route that sends traffic destined for subnet B to the appliance's IP (or NIC) as the next hop. Without this route, the VPC's default routing will deliver traffic directly between subnets, bypassing the appliance. Also, the appliance's NIC1 must be in the same VPC and the firewall rules must allow the traffic.
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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Last reviewed: Jul 4, 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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