NAT Gateway for private subnet internet in AWS - Step-by-Step Execution

Process Flow - NAT Gateway for private subnet internet

Private Subnet Instance

↓

Route Table: directs 0.0.0.0/0 to NAT Gateway

↓

NAT Gateway in Public Subnet

↓

Internet Gateway

↓

Internet

Traffic from private subnet instances goes to NAT Gateway via route table, then to Internet Gateway, enabling internet access without exposing private instances.

Execution Sample

AWS

1. Create NAT Gateway in public subnet
2. Update private subnet route table to send 0.0.0.0/0 to NAT Gateway
3. Private instances send internet traffic to NAT Gateway
4. NAT Gateway forwards traffic to Internet Gateway
5. Responses return back through NAT Gateway to private instances

This setup allows private subnet instances to access the internet securely via the NAT Gateway.

Process Table

Step	Action	Source	Destination	Route Used	Result
1	Instance sends request to internet	Private Subnet Instance	Internet IP	Route to NAT Gateway	Traffic sent to NAT Gateway
2	NAT Gateway receives traffic	NAT Gateway	Internet IP	Uses Internet Gateway	Traffic forwarded to Internet
3	Internet responds	Internet IP	NAT Gateway	Internet Gateway	Response received by NAT Gateway
4	NAT Gateway forwards response	NAT Gateway	Private Subnet Instance	Internal routing	Instance receives response
5	Instance processes response	Private Subnet Instance	Internal	N/A	Internet access successful
6	Instance tries direct internet access	Private Subnet Instance	Internet IP	No direct route	Fails - no direct internet access

💡 Private subnet instances cannot access internet directly; all traffic must go through NAT Gateway.

Status Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	Final
Instance Internet Access	No	Request sent	Request forwarded	Response received	Response forwarded	Yes

Key Moments - 3 Insights

Why can't private subnet instances access the internet directly?

Because their route table directs internet traffic to the NAT Gateway, not directly to the Internet Gateway, as shown in execution_table row 6.

What role does the NAT Gateway play in internet access?

It acts as a middleman forwarding traffic between private instances and the internet, enabling responses to return, as seen in execution_table rows 2, 3, and 4.

Why is the NAT Gateway placed in the public subnet?

Because it needs direct access to the Internet Gateway to forward traffic, which private subnets do not have, as shown in concept_flow.

Visual Quiz - 3 Questions

Look at the execution_table, at which step does the NAT Gateway forward the internet request to the Internet Gateway?

A Step 2

B Step 3

C Step 1

D Step 4

💡 Hint

Check the 'Action' and 'Result' columns in execution_table row 2.

According to variable_tracker, when does the instance first receive the internet response?

A After Step 2

B After Step 3

C After Step 4

D After Step 5

💡 Hint

Look at the 'Instance Internet Access' variable changes in variable_tracker after each step.

If the private subnet route table was changed to point 0.0.0.0/0 directly to the Internet Gateway, what would happen?

A Instances would lose internet access

B Instances would access internet directly

C NAT Gateway would still be used

D Internet Gateway would block traffic

💡 Hint

Refer to key_moments about routing and direct internet access.

Concept Snapshot

NAT Gateway for private subnet internet:
- Place NAT Gateway in public subnet with Elastic IP
- Private subnet route table directs 0.0.0.0/0 to NAT Gateway
- Private instances send internet traffic to NAT Gateway
- NAT Gateway forwards traffic via Internet Gateway
- Enables internet access without exposing private instances

Full Transcript

This visual execution shows how a NAT Gateway enables internet access for instances in a private subnet. The private subnet's route table sends all internet-bound traffic to the NAT Gateway located in a public subnet. The NAT Gateway then forwards this traffic to the Internet Gateway, which connects to the internet. Responses from the internet return through the Internet Gateway and NAT Gateway back to the private instances. Direct internet access from private instances fails because their route table does not point to the Internet Gateway. This setup secures private instances by not exposing them directly to the internet while allowing outbound internet connectivity.

Practice

1. What is the main purpose of a NAT Gateway in AWS for private subnets?

easy

A. To block all internet traffic from private subnets

B. To provide direct internet access to public subnets

C. To assign public IP addresses to private subnet instances

D. To allow instances in private subnets to access the internet securely

Solution

1. Step 1: Understand NAT Gateway role

   A NAT Gateway enables instances in private subnets to initiate outbound internet traffic while preventing inbound internet connections.

2. Step 2: Compare options

   Only To allow instances in private subnets to access the internet securely correctly describes this purpose; others describe unrelated or incorrect functions.

3. Final Answer:

   To allow instances in private subnets to access the internet securely -> Option D

4. Quick Check:

   NAT Gateway purpose = Allow private subnet internet access [OK]

Hint: NAT Gateway = private subnet internet access [OK]

Common Mistakes:

• Confusing NAT Gateway with Internet Gateway
• Thinking NAT Gateway assigns public IPs
• Believing NAT Gateway blocks internet traffic

2. Which AWS resource must a NAT Gateway be associated with to provide internet access for private subnets?

easy

A. An Elastic IP in a public subnet

B. A private IP in a private subnet

C. A security group allowing inbound SSH

D. A VPC endpoint for S3

Solution

1. Step 1: Identify NAT Gateway placement

   NAT Gateway must be placed in a public subnet and associated with an Elastic IP to route internet traffic.

2. Step 2: Evaluate options

   Only An Elastic IP in a public subnet correctly states the Elastic IP in a public subnet; others are unrelated or incorrect.

3. Final Answer:

   An Elastic IP in a public subnet -> Option A

4. Quick Check:

   NAT Gateway needs Elastic IP in public subnet [OK]

Hint: NAT Gateway needs Elastic IP in public subnet [OK]

Common Mistakes:

• Placing NAT Gateway in private subnet
• Associating NAT Gateway with private IP
• Confusing security groups with subnet placement

3. Given a private subnet with route table entry 0.0.0.0/0 -> NAT Gateway, what happens when an instance in this subnet tries to access the internet?

medium

A. Traffic is blocked because private subnets cannot access the internet

B. Traffic goes directly to the Internet Gateway

C. Traffic is routed through the NAT Gateway to the internet

D. Traffic is routed to the VPC endpoint

Solution

1. Step 1: Analyze route table entry

   The route 0.0.0.0/0 -> NAT Gateway directs all internet-bound traffic from the private subnet to the NAT Gateway.

2. Step 2: Understand traffic flow

   The NAT Gateway forwards traffic to the internet, allowing outbound access while hiding private IPs.

3. Final Answer:

   Traffic is routed through the NAT Gateway to the internet -> Option C

4. Quick Check:

   Route 0.0.0.0/0 to NAT Gateway = internet access [OK]

Hint: Route 0.0.0.0/0 to NAT Gateway = internet access [OK]

Common Mistakes:

• Assuming private subnet traffic goes directly to Internet Gateway
• Thinking private subnet traffic is blocked by default
• Confusing VPC endpoint with internet routing

4. You created a NAT Gateway in a public subnet but instances in the private subnet cannot access the internet. What is the most likely cause?

medium

A. The private subnet has a public IP assigned

B. The private subnet route table does not have a route to the NAT Gateway

C. The Internet Gateway is not attached to the VPC

D. The NAT Gateway is missing an Elastic IP

Solution

1. Step 1: Check route table configuration

   Instances in private subnets need a route directing internet traffic (0.0.0.0/0) to the NAT Gateway.

2. Step 2: Identify missing route issue

   If this route is missing, traffic won't reach the NAT Gateway, causing no internet access.

3. Final Answer:

   The private subnet route table does not have a route to the NAT Gateway -> Option B

4. Quick Check:

   Missing route to NAT Gateway = no internet [OK]

Hint: Check private subnet route table for NAT Gateway route [OK]

Common Mistakes:

• Ignoring route table routes
• Assuming Elastic IP is optional
• Confusing Internet Gateway attachment with NAT Gateway routing

5. You want to allow instances in a private subnet to access the internet for software updates but prevent inbound internet traffic. Which setup is correct?

hard

A. Place a NAT Gateway in a public subnet with an Elastic IP and route private subnet traffic (0.0.0.0/0) to it

B. Assign public IPs to private subnet instances and route traffic through the Internet Gateway

C. Place a NAT Gateway in the private subnet and route traffic to it

D. Use a VPC endpoint for internet access

Solution

1. Step 1: Understand NAT Gateway placement and function

   NAT Gateway must be in a public subnet with an Elastic IP to allow outbound internet access for private subnet instances without exposing them to inbound traffic.

2. Step 2: Evaluate options for security and functionality

   Place a NAT Gateway in a public subnet with an Elastic IP and route private subnet traffic (0.0.0.0/0) to it correctly describes this setup. Assign public IPs to private subnet instances and route traffic through the Internet Gateway exposes instances publicly. Place a NAT Gateway in the private subnet and route traffic to it places NAT Gateway incorrectly. Use a VPC endpoint for internet access does not provide internet access.

3. Final Answer:

   Place a NAT Gateway in a public subnet with an Elastic IP and route private subnet traffic (0.0.0.0/0) to it -> Option A

4. Quick Check:

   NAT Gateway in public subnet + route private subnet = secure internet access [OK]

Hint: NAT Gateway in public subnet with route from private subnet [OK]

Common Mistakes:

• Assigning public IPs to private subnet instances
• Placing NAT Gateway in private subnet
• Using VPC endpoint for general internet access