The answer is port exhaustion due to insufficient NAT IP addresses. With only one NAT IP assigned to the Cloud NAT gateway, all VM instances in the 10.1.0.0/24 subnet share a single public IP, which provides a maximum of 65,535 ephemeral source ports. During peak hours, each concurrent outbound TCP or UDP connection consumes a unique port; once all ports are in use, new connections are dropped, even though the gateway itself is healthy and firewall rules are correct. This scenario directly tests your understanding of Cloud NAT port exhaustion troubleshooting on the Google Professional Cloud Network Engineer exam, where a common trap is to suspect firewall rules or gateway health when the real bottleneck is port capacity. Remember the 65K ceiling: one NAT IP equals roughly 65,000 simultaneous connections, so if your subnet has hundreds of VMs making thousands of outbound calls, you need multiple NAT IPs or a NAT reservation to scale. Memory tip: think “one IP, 65K ports—exhaustion is the short report.”
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.
Network Topology
A company has a VPC with subnet 10.1.0.0/24 in us-central1. They created a Cloud NAT gateway named 'nat-us-central1' attached to that subnet. During peak hours, many VM instances in the subnet cannot connect to the internet. The NAT configuration shows only one NAT IP. Firewall rules allow egress traffic, and health checks confirm the NAT gateway is functioning. What is the most likely cause of the failure?
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.
With only one NAT IP address and many VM instances sharing it, the available source ports (65,535 per IP) are quickly exhausted during peak hours. Each concurrent outbound connection consumes a unique source port; once all ports are in use, new connections fail. This is a classic port exhaustion scenario, not a configuration or firewall issue.
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.
✗
The Cloud Router is missing or misconfigured.
Why it's wrong here
Cloud Router is required for NAT; the command output shows the NAT exists, indicating the router is present.
✗
The NAT gateway is not attached to the correct subnet.
Why it's wrong here
The exhibit shows the NAT is described, and the subnet is presumably attached.
✗
An egress firewall rule blocks ICMP packets.
Why it's wrong here
Port exhaustion affects all outbound connections, not just ICMP.
✓
Port exhaustion due to insufficient NAT IP addresses.
Why this is correct
One NAT IP provides limited source ports, easily exhausted by many concurrent connections.
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.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Google Cloud often tests the misconception that firewall rules or router misconfiguration are the primary cause of connectivity failures, when in reality port exhaustion from insufficient NATIPs is a common scaling issue in high-traffic environments.
Trap categories for this question
Command / output trap
Cloud Router is required for NAT; the command output shows the NAT exists, indicating the router is present.
Detailed technical explanation
How to think about this question
Cloud NAT uses source network address translation (SNAT) to map VM private IPs to a public NAT IP. Each TCP/UDP session consumes a unique 5-tuple (source IP, source port, destination IP, destination port, protocol). With a single NAT IP, the maximum concurrent outbound connections are limited to ~64,000 ports per destination IP/port pair. In production, you should allocate multiple NAT IPs (e.g., 2–5) or use a NAT IP pool to scale. The 'nat_ip_allocate_option' can be set to 'MANUAL_ONLY' or 'AUTO_ONLY' to control IP assignment.
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. — With only one NAT IP address and many VM instances sharing it, the available source ports (65,535 per IP) are quickly exhausted during peak hours. Each concurrent outbound connection consumes a unique source port; once all ports are in use, new connections fail. This is a classic port exhaustion scenario, not a configuration or firewall issue.
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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Question Discussion
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