- A
Set the MTU to 1500 on both interfaces
Why wrong: GCP's default MTU is 1460, but 1500 is not required for routing.
- B
Create a route with a next-hop of the instance's internal IP
Why wrong: Routes are needed to direct traffic to the appliance, but the question asks for actions on the instance itself.
- C
Enable IP forwarding on the instance
IP forwarding allows the instance to forward packets between interfaces.
- D
Attach two network interfaces to the instance, one in each subnet
Multiple NICs allow the instance to be on multiple subnets.
- E
Assign external IPs to both interfaces
Why wrong: External IPs are not required for internal routing.
Question 254 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.
An engineer needs to configure a Compute Engine instance as a network appliance that routes traffic between two subnets within the same VPC. The instance must handle traffic for both subnets. Which TWO actions are required? (Choose TWO.)
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
✓
Enable IP forwarding on the instance
To act as a router between subnets, the instance needs two network interfaces, each attached to a different subnet, and IP forwarding must be enabled so the kernel can forward packets between interfaces.
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.
- ✗
Set the MTU to 1500 on both interfaces
Why it's wrong here
GCP's default MTU is 1460, but 1500 is not required for routing.
- ✗
Create a route with a next-hop of the instance's internal IP
Why it's wrong here
Routes are needed to direct traffic to the appliance, but the question asks for actions on the instance itself.
- ✓
Enable IP forwarding on the instance
Why this is correct
IP forwarding allows the instance to forward packets between interfaces.
Related concept
CIDR notation defines the prefix length.
- ✓
Attach two network interfaces to the instance, one in each subnet
Why this is correct
Multiple NICs allow the instance to be on multiple subnets.
Related concept
CIDR notation defines the prefix length.
- ✗
Assign external IPs to both interfaces
Why it's wrong here
External IPs are not required for internal routing.
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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Implementing VPC Instances practice questions
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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: Enable IP forwarding on the instance — To act as a router between subnets, the instance needs two network interfaces, each attached to a different subnet, and IP forwarding must be enabled so the kernel can forward packets between interfaces.
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.
About these practice questions
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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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