Cloud NAT for private instances in GCP - Step-by-Step Execution

Process Flow - Cloud NAT for private instances

Private VM without public IP

↓

Send outbound traffic

↓

Traffic reaches Cloud NAT

↓

Cloud NAT translates private IP to NAT IP

↓

Traffic goes to internet

↓

Response returns to Cloud NAT

↓

Cloud NAT translates NAT IP back to private IP

↓

Response delivered to VM

Private VMs send outbound traffic which Cloud NAT translates to a public IP, enabling internet access without exposing VM's private IP.

Execution Sample

GCP

resource "google_compute_router" "nat-router" {
  name    = "nat-router"
  network = "default"
  region  = "us-central1"
}

resource "google_compute_router_nat" "nat-config" {
  name   = "nat-config"
  router = google_compute_router.nat-router.name
  region = "us-central1"

  nat_ip_allocate_option = "AUTO_ONLY"
  source_subnetwork_ip_ranges_to_nat = "ALL_SUBNETWORKS_ALL_IP_RANGES"
}

Terraform config creates a Cloud Router and Cloud NAT to enable private VMs in all subnetworks to access the internet.

Process Table

Step	Action	Source IP	NAT Translation	Destination	Response Path
1	VM sends outbound request	10.128.0.5 (private)	No translation yet	Internet service	N/A
2	Packet reaches Cloud NAT	10.128.0.5 (private)	Translate to NAT IP (e.g., 35.192.0.1)	Internet service	N/A
3	Internet service receives request	35.192.0.1 (NAT IP)	N/A	Responds to NAT IP	N/A
4	Response arrives at Cloud NAT	35.192.0.1 (NAT IP)	Translate back to 10.128.0.5 (private)	VM	Forward response
5	VM receives response	10.128.0.5 (private)	No translation	VM	Complete communication
6	No inbound unsolicited traffic allowed	N/A	N/A	N/A	N/A

💡 Communication ends after response is delivered to VM; inbound unsolicited traffic blocked by NAT.

Status Tracker

Variable	Start	After Step 2	After Step 4	Final
Source IP	10.128.0.5	35.192.0.1 (NAT IP)	10.128.0.5	10.128.0.5
Destination	Internet service	Internet service	VM	VM
Response Path	N/A	N/A	Forward response	Complete communication

Key Moments - 3 Insights

Why does the VM not need a public IP to access the internet?

Because Cloud NAT translates the VM's private IP to a public NAT IP for outbound traffic, as shown in execution_table steps 2 and 4.

Can inbound unsolicited traffic from the internet reach the VM through Cloud NAT?

No, Cloud NAT only allows responses to outbound requests; unsolicited inbound traffic is blocked, as noted in execution_table step 6.

What happens to the source IP of packets when they leave the VM and reach the internet?

The source IP is translated from the VM's private IP to the NAT IP by Cloud NAT, as shown in execution_table step 2.

Visual Quiz - 3 Questions

Look at the execution_table, what is the source IP seen by the internet service at step 3?

A 10.128.0.5 (private IP)

B 35.192.0.1 (NAT IP)

C Unknown IP

D VM's public IP

💡 Hint

Refer to execution_table row for step 3 under 'Source IP' column.

At which step does Cloud NAT translate the response back to the VM's private IP?

A Step 2

B Step 3

C Step 4

D Step 5

💡 Hint

Check execution_table row for step 4 under 'NAT Translation' column.

If the VM had a public IP, how would the execution_table change at step 2?

A No NAT translation needed, source IP remains VM's public IP

B Traffic would be blocked

C Cloud NAT would still translate the IP

D Response path would be different

💡 Hint

Consider the role of Cloud NAT in translating private IPs only.

Concept Snapshot

Cloud NAT enables private VMs without public IPs to access the internet.
It translates private IPs to public NAT IPs for outbound traffic.
Inbound unsolicited traffic is blocked for security.
Configured via Cloud Router and NAT resource.
Supports all subnetworks or selected ranges.
Ensures private instances stay secure while accessing external services.

Full Transcript

Cloud NAT allows virtual machines without public IP addresses to access the internet by translating their private IP addresses to a public NAT IP. When a VM sends outbound traffic, it reaches Cloud NAT, which replaces the private IP with a NAT IP before sending it to the internet. Responses from the internet come back to Cloud NAT, which translates the NAT IP back to the VM's private IP and forwards the response. This process enables internet access without exposing the VM's private IP. Cloud NAT blocks inbound unsolicited traffic, enhancing security. The setup involves creating a Cloud Router and configuring Cloud NAT to handle IP translation for private instances.

Practice

1. What is the main purpose of Cloud NAT for private instances in Google Cloud?

easy

A. Create a VPN connection between private instances

B. Assign public IP addresses to all instances automatically

C. Allow private instances to access the internet without public IP addresses

D. Enable inbound internet traffic to private instances

Solution

1. Step 1: Understand Cloud NAT's role

   Cloud NAT allows private instances to send outbound traffic to the internet without needing public IPs.

2. Step 2: Eliminate incorrect options

   Assigning public IPs automatically, creating a VPN connection between private instances, and enabling inbound internet traffic to private instances describe different functions not provided by Cloud NAT.

3. Final Answer:

   Allow private instances to access the internet without public IP addresses -> Option C

4. Quick Check:

   Cloud NAT = outbound internet without public IP [OK]

Hint: Cloud NAT = outbound internet for private instances [OK]

Common Mistakes:

• Confusing Cloud NAT with assigning public IPs
• Thinking Cloud NAT allows inbound internet traffic
• Mixing Cloud NAT with VPN functionality

2. Which of the following is the correct way to configure Cloud NAT in Google Cloud Console?

easy

A. Create a firewall rule allowing inbound NAT traffic

B. Create a Cloud Router, then add a Cloud NAT configuration linked to it

C. Enable Cloud NAT on the VM instance network interface

D. Directly assign a public IP to each private instance

Solution

1. Step 1: Identify Cloud NAT setup steps

   Cloud NAT requires a Cloud Router to manage NAT configurations.

2. Step 2: Review options for correctness

   Create a Cloud Router, then add a Cloud NAT configuration linked to it correctly describes creating a Cloud Router and linking Cloud NAT. Assigning public IPs to private instances and enabling NAT on VM interfaces directly are incorrect because Cloud NAT does not work that way. Creating a firewall rule allowing inbound NAT traffic is wrong because NAT does not require inbound firewall rules.

3. Final Answer:

   Create a Cloud Router, then add a Cloud NAT configuration linked to it -> Option B

4. Quick Check:

   Cloud NAT setup = Cloud Router + NAT config [OK]

Hint: Cloud NAT always needs a Cloud Router first [OK]

Common Mistakes:

• Trying to assign public IPs to private instances
• Enabling NAT directly on VM interfaces
• Confusing firewall rules with NAT configuration

3. Given this snippet of a Cloud NAT configuration in Terraform, what will happen if private instances try to access the internet?

resource "google_compute_router" "router" {
  name    = "my-router"
  network = "default"
  region  = "us-central1"
}

resource "google_compute_router_nat" "nat" {
  name                               = "my-nat"
  router                             = google_compute_router.router.name
  region                             = "us-central1"
  nat_ip_allocate_option             = "AUTO_ONLY"
  source_subnetwork_ip_ranges_to_nat = "ALL_SUBNETWORKS_ALL_IP_RANGES"
}

medium

A. Private instances will access the internet using automatically allocated NAT IPs

B. Private instances will fail to access the internet due to missing public IPs

C. Only instances with public IPs will access the internet

D. The NAT configuration will block all outbound traffic

Solution

1. Step 1: Analyze Terraform NAT configuration

   The NAT uses AUTO_ONLY for IP allocation and applies to all subnetworks and IP ranges, allowing private instances to use NAT IPs automatically.

2. Step 2: Determine expected behavior

   Since NAT IPs are auto-allocated and all subnetworks are included, private instances will access the internet through NAT IPs.

3. Final Answer:

   Private instances will access the internet using automatically allocated NAT IPs -> Option A

4. Quick Check:

   Auto NAT IPs + all subnetworks = internet access [OK]

Hint: AUTO_ONLY means NAT IPs assigned automatically [OK]

Common Mistakes:

• Assuming private instances need public IPs
• Thinking NAT blocks outbound traffic
• Confusing source_subnetwork_ip_ranges_to_nat setting

4. You configured Cloud NAT but private instances still cannot access the internet. What is the most likely cause?

medium

A. The instances have public IPs assigned

B. Cloud NAT is configured with AUTO_ONLY IP allocation

C. Firewall rules allow outbound traffic

D. The Cloud Router is not in the same region as the instances

Solution

1. Step 1: Check Cloud NAT regional requirements

   Cloud NAT and Cloud Router must be in the same region as the instances to route traffic correctly.

2. Step 2: Evaluate other options

   The instances have public IPs assigned is not a problem; public IPs are optional. Firewall rules allow outbound traffic is good practice, not a cause of failure. Cloud NAT is configured with AUTO_ONLY IP allocation is a valid IP allocation method.

3. Final Answer:

   The Cloud Router is not in the same region as the instances -> Option D

4. Quick Check:

   Cloud Router region mismatch blocks NAT [OK]

Hint: Cloud Router and NAT must match instance region [OK]

Common Mistakes:

• Assuming public IPs are required for NAT
• Ignoring region mismatch issues
• Confusing firewall rules with NAT problems

5. You want to allow private instances in multiple subnetworks across different regions to access the internet using Cloud NAT. Which setup is best practice?

hard

A. Create a Cloud Router and Cloud NAT in each region, linked to subnetworks in that region

B. Create one Cloud Router and Cloud NAT in a single region for all subnetworks

C. Assign public IPs to all instances instead of using Cloud NAT

D. Use a single Cloud Router with multiple NAT configurations in one region

Solution

1. Step 1: Understand Cloud NAT regional scope

   Cloud NAT and Cloud Router are regional resources; they must be created in each region where subnetworks exist.

2. Step 2: Evaluate options for multi-region setup

   Create a Cloud Router and Cloud NAT in each region, linked to subnetworks in that region correctly creates routers and NATs per region. Create one Cloud Router and Cloud NAT in a single region for all subnetworks and using a single Cloud Router with multiple NAT configurations in one region incorrectly assume cross-region coverage. Assigning public IPs to all instances instead of using Cloud NAT avoids NAT but is less secure and not best practice.

3. Final Answer:

   Create a Cloud Router and Cloud NAT in each region, linked to subnetworks in that region -> Option A

4. Quick Check:

   Cloud NAT is regional, so multi-region needs multiple routers [OK]

Hint: Cloud NAT is regional; create per region needed [OK]

Common Mistakes:

• Trying to use one NAT for all regions
• Assigning public IPs instead of NAT
• Misunderstanding Cloud NAT regional limits