The answer is port exhaustion due to insufficient NAT IP addresses. Cloud NAT relies on source network address translation (SNAT) to map private VM IPs to a single public IP, and each NAT IP provides only 64,512 ephemeral source ports per VM for concurrent outbound connections. When egress traffic from Compute Engine VMs spikes, these ports are consumed rapidly, and once exhausted, new connections are dropped, causing the intermittent failures described. On the Google Professional Cloud Network Engineer exam, this scenario tests your understanding of Cloud NAT port limits and the relationship between NAT IP count and scalability—a common trap is to blame firewall rules or routing instead of the single IP bottleneck. Remember the memory tip: one IP equals roughly 64K ports per VM, so if traffic is spotty and you have only one address, think ports, not packets.
PCNE Configuring network services Practice Question
This PCNE practice question tests your understanding of configuring network services. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. 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 uses Cloud NAT to allow private instances to reach the internet. They notice that egress traffic from Compute Engine VMs is intermittently failing. The VMs are in us-central1-a and use the default VPC network. Cloud NAT is configured with a single NAT IP address. What is the most likely cause?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Port exhaustion due to insufficient NAT IP addresses
Cloud NAT uses source network address translation (SNAT) to map private VMIPs to a public NAT IP. With only a single NAT IP address, the available port range (typically 64,512 ephemeral ports per NAT IP per VM) can be exhausted under high egress traffic, causing intermittent failures. This is the most likely cause given the symptom of intermittent failures and the single IP configuration.
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.
Port exhaustion due to insufficient NAT IP addresses
Why this is correct
A single NAT IP provides limited ports; many VMs can exhaust them.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
Cloud NAT not configured in the correct region
Why it's wrong here
Cloud NAT is regional; VMs in us-central1-a are in the same region.
✗
Firewall rule blocking egress traffic from VM
Why it's wrong here
Default VPC allows egress; no evidence of custom rules.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Google Cloud often tests the misconception that Cloud NAT automatically scales with traffic or that a single NAT IP is sufficient for any workload, when in reality port exhaustion is a common scaling bottleneck.
Detailed technical explanation
How to think about this question
Cloud NAT allocates a pool of ephemeral source ports from the NAT IP's port range (1–65535, minus reserved ports). Each concurrent connection to a unique destination (IP:port) consumes one port; when all ports are in use, new connections fail until ports are released (TCP TIME_WAIT or UDP timeout). In practice, a single NAT IP supports roughly 64,512 simultaneous connections per VM, but bursty traffic or many short-lived connections can quickly exhaust this, causing intermittent failures.
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
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.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Configuring network services — This question tests Configuring network services — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Port exhaustion due to insufficient NAT IP addresses — Cloud NAT uses source network address translation (SNAT) to map private VM IPs to a public NAT IP. With only a single NAT IP address, the available port range (typically 64,512 ephemeral ports per NAT IP per VM) can be exhausted under high egress traffic, causing intermittent failures. This is the most likely cause given the symptom of intermittent failures and the single IP configuration.
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.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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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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