Internet Gateway (IGW) | Overview.

An Overview of Internet Gateway (IGW) in AWS.

Scope:

•         The concept: Internet Gateway,
•        Core Functions of an IGW,
•        How Internet Gateway Works,
•        Configuration Sample,
•        Traffic Flow Sample (Public Instance)
•        Contrast: IGW vs NAT Gateway,
•        Advanced Notes,
•        Visual Diagram for Internet Gateway.

 The concept: Internet Gateway

•        An Internet Gateway (IGW) is a AWS-managed component that allows communication between twtech VPC and the public Internet.
•        An Internet Gateway (IGW) is horizontally scaled, redundant, and highly available.

• An Internet Gateway (IGW)  serves two main purposes:

1. Outbound access: Enables instances in public subnets to send traffic to the Internet.
2. Inbound access: Allows Internet-based clients to reach instances in public subnets (if permitted by routing and security rules).

 Core Functions of an IGW

Function	Description
Traffic translation	Translates private IP traffic (via Elastic IPs or public IPs) for Internet communication.
Routing target	Acts as a target for public routes in the VPC route table (0.0.0.0/0 → IGW).
Bidirectional communication	Enables both inbound and outbound Internet communication (if routing, NACLs, and SGs allow).
Elastic scaling	AWS automatically scales IGWs to handle traffic loads—no provisioning or scaling required.
No cost	There’s no cost for creating or using an IGW (twtech pays only for data transfer).

 How Internet Gateway Works

1, Public Subnet Association

A subnet is public when:

• Its route table has a default route (0.0.0.0/0) that targets the Internet Gateway.

2, Instance Accessibility

An EC2 instance in a public subnet can reach the Internet if:

• It has a public IPv4 address or Elastic IP.
• The route table has a default route to IGW.
• Security groups and NACLs allow outbound traffic.

3, Return Traffic

NB:

Return traffic is automatically routed back through the IGW (AWS tracks connection state).

 Configuration Sample: Step-by-Step Setup

1. Create/Attach IGW

# bash

 

aws ec2 create-internet-gateway

aws ec2 attach-internet-gateway --vpc-id vpc-xxxxxx --internet-gateway-id igw-xxxxxx

2. Update Route Table
• Public route table → 0.0.0.0/0 → Internet Gateway (igw-xxxxxx)
3. Assign Public IP or EIP
• EC2 → Networking → “Auto-assign Public IPv4 address: Enabled”
• or associate an Elastic IP manually.
4. Verify Security
• Security Group: Allow outbound (0.0.0.0/0) and inbound (e.g., port 22 or 80) if needed.
• NACLs: Ensure inbound/outbound rules allow traffic.

 Traffic Flow Sample (Public Instance)

Inbound Path (Internet → Instance):

 Contrast: IGW vs NAT Gateway

Feature	Internet Gateway	NAT Gateway
Purpose	Public Internet access (bidirectional)	Outbound-only Internet access for private subnets
Subnet type	Used with public subnets	Used with private subnets
IP mapping	Uses public/elastic IPs	Uses NAT EIP
Inbound traffic	Allowed (if configured)	Not allowed
Outbound traffic	Allowed	Allowed (for private subnets)

 Advanced Notes

• One IGW per VPC (though twtech can detach and reattach to another VPC).
• Supports IPv6 natively (no NAT required).
• Highly available across all Availability Zones automatically.
• Not stateful in itself — relies on VPC stateful rules and connection tracking.

 Visual Diagram for Internet Gateway