PCNE Configuring Network Services Practice Question
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?
⚠ Common exam trap
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 NAT IPs is a common scaling issue in high-traffic environments.
Answer choices
Why each option matters
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.
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.
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JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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.