The answer is that the default behavior for traffic between subnets in the same VPC is that it is allowed by the implicit default firewall rule, which permits all internal traffic. This works because a VPC is a global, isolated network, and subnets within it are simply IP address ranges that share the same routing table; by default, the VPC’s implied internal firewall rule allows any instance to communicate with any other instance using private IPs, regardless of subnet. On the Google Professional Cloud Network Engineer exam, this concept tests your understanding of VPC fundamentals versus common misconceptions—many candidates mistakenly think you need a custom firewall rule or VPC peering for intra-VPC subnet traffic, but the trap is that no additional configuration is required. A strong memory tip: think of a VPC as a single house with multiple rooms (subnets); the doors between rooms are always open by default, so you don’t need a key (firewall rule) to walk from one room to another.
PCNE Implementing a Virtual Private Cloud Practice Question
This PCNE practice question tests your understanding of implementing a virtual private cloud. Match the stated requirement to the specific cloud service, access model, or configuration option — many options are valid in isolation but not for this scenario. 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 developer needs to allow a VM in subnet A to reach a VM in subnet B in the same VPC. What is the default behavior?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The default firewall rule allows all internal traffic within the VPC.
By default, VPC networks allow all internal traffic between subnets. Option B correctly states this. Option A is wrong because no firewall rule is needed for internal traffic. Option C is wrong because VPC peering is for cross-VPC connectivity. Option D is wrong because custom routes are not required for internal subnet communication.
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.
✗
A custom route must be added to route between subnets.
Why it's wrong here
No custom routes are needed; VPC automatically routes within the network.
✗
A firewall rule must be added to allow traffic between subnets.
VPC peering is required for communication between subnets.
Why it's wrong here
VPC peering is for connectivity between different VPCs, not within the same VPC.
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.
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 default firewall rule allows all internal traffic within the VPC. — By default, VPC networks allow all internal traffic between subnets. Option B correctly states this. Option A is wrong because no firewall rule is needed for internal traffic. Option C is wrong because VPC peering is for cross-VPC connectivity. Option D is wrong because custom routes are not required for internal subnet communication.
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.
Question Discussion
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