Understanding Internet Gateway vs. NAT Gateway

1. Internet Gateway An Internet Gateway (IGW) allows instances in your public subnets to:

• Access the internet.
• Be accessed from the internet (e.g., SSH from your computer to a public EC2 instance).

Key Characteristics:

• Direct connection between your VPC and the internet.
• Required for instances that need public IP addresses for external communication.
• Outbound and inbound traffic is supported.

When to Use:

• When you need instances in a public subnet to be accessible from the internet.
• Examples:
  • Hosting a public-facing web server.
  • SSHing into a public EC2 instance.

Steps to Configure:

1. Attach an Internet Gateway to your VPC:

   resource "aws_internet_gateway" "example" {
     vpc_id = aws_vpc.example.id
   
     tags = {
       Name = "example-internet-gateway"
     }
   }

2. Add a route in the route table for the public subnet to direct traffic to the internet:

   resource "aws_route_table" "public_route_table" {
     vpc_id = aws_vpc.example.id
   
     route {
       cidr_block = "0.0.0.0/0"
       gateway_id = aws_internet_gateway.example.id
     }
   
     tags = {
       Name = "PublicRouteTable"
     }
   }
   
   resource "aws_route_table_association" "public_association" {
     subnet_id      = aws_subnet.public.id
     route_table_id = aws_route_table.public_route_table.id
   }

3. Ensure the instance has a public IP and security group allowing inbound access (e.g., SSH):

   associate_public_ip_address = true

4. NAT Gateway A NAT Gateway allows instances in your private subnets to:

• Initiate outbound internet traffic (e.g., downloading updates or software).
• Prevent direct inbound access from the internet.

Key Characteristics:

• Provides internet access for private subnets.
• Outbound traffic is supported; inbound traffic is not allowed.
• Requires an Elastic IP (EIP).

When to Use:

• When you need instances in a private subnet to communicate with the internet without being exposed to the internet.
• Examples:
  • EC2 instances in a private subnet needing to download updates or connect to external APIs.
  • Database servers fetching security patches.

Steps to Configure:

1. Create a NAT Gateway in a public subnet:

   resource "aws_nat_gateway" "example" {
     allocation_id = aws_eip.example.id
     subnet_id     = aws_subnet.public.id
   
     tags = {
       Name = "example-nat-gateway"
     }
   }

2. Add a route in the route table for the private subnet to direct traffic to the NAT Gateway:

   resource "aws_route_table" "private_route_table" {
     vpc_id = aws_vpc.example.id
   
     route {
       cidr_block = "0.0.0.0/0"
       nat_gateway_id = aws_nat_gateway.example.id
     }
   
     tags = {
       Name = "PrivateRouteTable"
     }
   }
   
   resource "aws_route_table_association" "private_association" {
     subnet_id      = aws_subnet.private.id
     route_table_id = aws_route_table.private_route_table.id
   }

Key Differences and When to Use Each:

Feature	Internet Gateway (IGW)	NAT Gateway
Purpose	Connect public subnets to the internet	Allow private subnets to access the internet
Inbound Traffic	Allowed for instances with public IP	Not allowed (private instances are not exposed)
Outbound Traffic	Allowed for instances with public IP	Allowed for instances in private subnets
Use Case	Public-facing services, SSH access	Secure private instances needing outbound access
Subnet Placement	Attached to public subnets	Placed in public subnets to serve private subnets

How to Remember?

• Internet Gateway = Public Access: Think of it as the "door" to the internet for instances that need to be accessible from the outside.
• NAT Gateway = Private Access: Think of it as the "proxy" that lets private instances talk to the internet but hides them from incoming traffic.

Your Situation: For SSH access from your computer to the public EC2 instance:

• Use an Internet Gateway.
• Ensure:
  1. The EC2 instance is in a public subnet.
  2. The EC2 instance has a public IP.
  3. Security group allows SSH (port 22) access from your computer's IP address.

For private EC2 instances needing updates or internet access:

• Use a NAT Gateway.

	•	A NAT Gateway enables outbound internet access from private subnets. It does not itself allow inbound SSH from the internet into private subnets.
	•	To SSH from your local laptop into an instance via the public internet, that instance must be in a public subnet (with a route to the Internet Gateway) and must have a public IP or an Elastic IP.