The answer is to apply a network tag to instances that should use NAT and create a Cloud NAT gateway configured with that specific tag. This works because Cloud NAT uses network tags as a filtering mechanism, allowing you to define which virtual machine instances are eligible for outbound NAT through the gateway; only instances bearing the designated tag will have their traffic translated, while untagged instances bypass the gateway entirely. On the Google Professional Cloud Network Engineer exam, this concept tests your understanding of Cloud NAT’s tag-based routing, often appearing in scenario-based questions where you must distinguish between enabling NAT for all instances versus selective access. A common trap is assuming that firewall rules or subnet-level settings control NAT eligibility, but Cloud NAT’s tag filter is the precise tool. Memory tip: think of the tag as a “NAT pass” — only tagged VMs get to use the gateway, just like a backstage pass at a concert.
PCNE Implementing a Virtual Private Cloud Practice Question
This PCNE practice question tests your understanding of implementing a virtual private cloud. 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 network engineer wants to allow specific instances to use Cloud NAT while others should not. Which configuration step should be taken?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Apply a network tag to instances that should use NAT and create a Cloud NAT with that tag.
Cloud NAT uses network tags to filter which instances use it.
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.
✗
Assign external IPs to non-NAT instances.
Why it's wrong here
Instances with external IPs bypass Cloud NAT, but this is not the best practice for selective use.
✗
Use firewall rules to allow NAT for tagged instances.
Why it's wrong here
Firewall rules do not control which instances use Cloud NAT.
✓
Apply a network tag to instances that should use NAT and create a Cloud NAT with that tag.
Why this is correct
This is the standard method to select which instances are NATed.
Cloud NAT works at the subnet level, but this is not the recommended way to filter instances.
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.
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: Apply a network tag to instances that should use NAT and create a Cloud NAT with that tag. — Cloud NAT uses network tags to filter which instances use it.
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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Question Discussion
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