The correct answer is to remove any NAT rules that match the source subnet and include external IP addresses, because Cloud NAT’s default behavior is to only apply source network address translation to VMs without external IPs. This default exists because a VM with an external IP already has a public address for outbound communication, so Cloud NAT will not interfere unless you explicitly configure NAT rules to override that behavior. On the Google Professional Cloud Network Engineer exam, this concept tests your understanding of Cloud NAT’s implicit priority: it respects a VM’s existing external IP unless you deliberately force NAT via custom rules. A common trap is assuming Cloud NAT always applies to all VMs in a subnet, but the key distinction is that external IPs are the default escape hatch. Memory tip: think “NAT only for the NAT-less” — if a VM has its own public IP, Cloud NAT steps aside.
PCNE Implementing network security Practice Question
This PCNE practice question tests your understanding of implementing network security. 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 multiple VPC networks in the same project, each with its own Cloud NAT configuration. They notice that traffic from a VM in VPC-A that has an external IP address is being NATed through the Cloud NAT gateway, but they only want Cloud NAT to be used for VMs without external IPs. What configuration ensures this?
Disable the 'send-tosource-ip' flag on the Cloud NAT gateway.
Why wrong: This flag controls whether packets preserve the source IP; does not prevent NAT for VMs with external IPs.
B
Set the network tier of the VM to 'Standard' instead of 'Premium'.
Why wrong: Network tier does not affect Cloud NAT behavior.
C
Remove any NAT rules that match the source subnet and include external IP addresses; Cloud NAT automatically applies only to VMs without external IPs if not explicitly configured otherwise.
By default, Cloud NAT only applies to VMs without external IPs when using default configuration.
D
Enable Cloud NAT logging to track which VMs are NATed.
Why wrong: Logging only provides visibility, does not change behavior.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Remove any NAT rules that match the source subnet and include external IP addresses; Cloud NAT automatically applies only to VMs without external IPs if not explicitly configured otherwise.
Option C is correct because Cloud NAT only performs source network address translation for VMs that do not have external IP addresses, unless you explicitly configure NAT rules that match traffic from VMs with external IPs. By removing any such custom NAT rules, the default behavior ensures that only VMs without external IPs are NATed through the Cloud NAT gateway, leaving VMs with external IPs to use their own public addresses directly.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✗
Disable the 'send-tosource-ip' flag on the Cloud NAT gateway.
Why it's wrong here
This flag controls whether packets preserve the source IP; does not prevent NAT for VMs with external IPs.
✗
Set the network tier of the VM to 'Standard' instead of 'Premium'.
Remove any NAT rules that match the source subnet and include external IP addresses; Cloud NAT automatically applies only to VMs without external IPs if not explicitly configured otherwise.
Why this is correct
By default, Cloud NAT only applies to VMs without external IPs when using default configuration.
Related concept
Read the scenario before looking for a memorised answer.
✗
Enable Cloud NAT logging to track which VMs are NATed.
Why it's wrong here
Logging only provides visibility, does not change behavior.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates assume Cloud NAT always NATs all VMs in a subnet, but the default behavior explicitly excludes VMs with external IPs unless custom NAT rules are added to include them.
Detailed technical explanation
How to think about this question
Cloud NAT uses a set of NAT rules defined in a Cloud Router configuration; by default, it only NATs traffic from VMs without external IPs (i.e., VMs that use only internal IPs). If you add a custom NAT rule that matches a source subnet containing VMs with external IPs, those VMs will also be NATed, overriding the default exclusion. This behavior is important in hybrid cloud scenarios where you might want to force all outbound traffic through a single NAT IP for logging or compliance, but if not needed, the default rule should be left untouched.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
TExam Day Tips
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A media company stores terabytes of video archives that are accessed once a year for audit purposes. Moving these objects to a cold storage tier (Azure Archive, S3 Glacier, or Google Nearline) costs a fraction of hot storage. Questions like this test whether you understand storage tiers, access frequency tradeoffs, and retrieval latency requirements.
What to study next
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Implementing network security — This question tests Implementing network security — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Remove any NAT rules that match the source subnet and include external IP addresses; Cloud NAT automatically applies only to VMs without external IPs if not explicitly configured otherwise. — Option C is correct because Cloud NAT only performs source network address translation for VMs that do not have external IP addresses, unless you explicitly configure NAT rules that match traffic from VMs with external IPs. By removing any such custom NAT rules, the default behavior ensures that only VMs without external IPs are NATed through the Cloud NAT gateway, leaving VMs with external IPs to use their own public addresses directly.
What should I do if I get this PCNE question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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