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Vijaya Laxmi Kadham
Vijaya Laxmi Kadham

Posted on • Originally published at my-cloud-journey.hashnode.dev

AWS Internet Gateway and Route Tables Explained for Beginners

#cloudcomputing #aws #beginners #learning

After learning about Public Subnets and Private Subnets, the next question that comes across our mind is:

How does traffic actually move inside AWS?

Creating a subnet alone doesn't make your application accessible.

AWS needs networking concepts to decide:

  • Where the traffic comes from
  • Where the traffic should go
  • Whether internet access is allowed

This is where Internet Gateways (IGW) and Route Tables come into the picture.

Together, they act like roads and traffic signals for your AWS network.

Imagine a Real City

Think of your AWS VPC as a city.

Inside the city, we have:

  • Buildings = EC2 Instances
  • Neighborhoods = Subnets
  • Roads = Routes
  • City Gate = Internet Gateway

Without roads and a city gate:

  • Nobody can enter
  • Nobody can leave

AWS networking works in a similar way.

Real City 

City
 ├── Buildings
 ├── Roads
 └── Main Gate
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AWS Equivalent 

VPC
 ├── EC2 Instances
 ├── Route Tables
 └── Internet Gateway
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What is an Internet Gateway?

An Internet Gateway (IGW) is a VPC component that enables communication between:

  • Your AWS VPC
  • The Public Internet

It is the official entry and exit point for internet traffic.

Real-World Example

Imagine a shopping mall.

It has:

  • Shops inside
  • Customers outside

Customers can only enter through the main entrance gate.

Customers
     ↓
 Main Entrance
     ↓
 Shopping Mall
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In AWS we can look the above situation as:

Internet
     ↓
Internet Gateway
     ↓
VPC
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The Internet Gateway acts as the main entrance gate.

Without an Internet Gateway

Suppose you launch an EC2 instance. But there is no Internet Gateway attached.

EC2 Instance
    ↓
Public Subnet
    ↓
VPC
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Result

  • ❌ Users cannot access the application
  • ❌ EC2 cannot browse the internet
  • ❌ Software updates cannot be downloaded

Even though the EC2 instance exists, it is isolated from the internet.

When an Internet Gateway is Attached 

Internet
     ↓
Internet Gateway
     ↓
VPC
     ↓
EC2
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The VPC now has a connection to the outside world.

However, there is still one more requirement.

The traffic needs directions.

This is where Route Tables come in.

What is a Route Table?

A Route Table is a set of rules that tells AWS:

"Where should the traffic go?"

Think of it like Google Maps for network traffic.

When traffic arrives, AWS checks the Route Table and decides whether to:

  • Send traffic to an Internet Gateway
  • Send traffic to another subnet
  • Send traffic to another network

Real-World Example

Imagine you're driving a car.

When you reach an intersection, you see:

Go Left  → Airport
Go Right → City Center
Go Straight → Highway
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Road signs tell you where to go.

A Route Table does the same thing for network traffic.

Understanding Routes

A route contains:

Destination → Target
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Example:

0.0.0.0/0 → Internet Gateway
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This means:

Any traffic going anywhere on the internet should be sent to the Internet Gateway.

What Does 0.0.0.0/0 Mean?

This confuses many beginners.

0.0.0.0/0
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means:

Every possible IP address on the internet.

So this route means:

All Internet Traffic
          ↓
Internet Gateway
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Complete Traffic Flow

Let's see what happens when a user opens a website.

Step 1

User enters:

www.example.com
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Step 2

The request reaches AWS.

User
 ↓
Internet
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Step 3

Traffic enters through the Internet Gateway.

User
 ↓
Internet
 ↓
Internet Gateway
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Step 4

AWS checks the Route Table.

Internet Gateway
        ↓
    Route Table
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Step 5

The Route Table sends traffic to the correct subnet.

Route Table
      ↓
Public Subnet
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Step 6

Traffic reaches the EC2 instance.

Public Subnet
      ↓
EC2 Instance
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Complete Flow 

User
  ↓
Internet
  ↓
Internet Gateway
  ↓
Route Table
  ↓
Public Subnet
  ↓
EC2 Instance
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Public Route Table Example

A Public Subnet becomes public because its Route Table contains a route to the Internet Gateway.

Destination Target
VPC CIDR Local
0.0.0.0/0 Internet Gateway

Diagram:

Public Subnet
       ↓
Route Table
       ↓
0.0.0.0/0
       ↓
Internet Gateway
       ↓
Internet
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This allows:

  • Incoming internet traffic
  • Outgoing internet traffic

Important Thing to Remember

Many beginners think:

"If an EC2 is launched inside a public subnet, it automatically becomes public."

This is incorrect.

For an EC2 instance to be publicly accessible, it needs:

Requirement 1

The subnet must have a route:

0.0.0.0/0 → IGW
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Requirement 2

An Internet Gateway must be attached.

Requirement 3

The EC2 instance must have a Public IP.

Requirement 4

The Security Group must allow access.

Only then can internet users reach the EC2 instance.

Private Subnet Route Table

A Private Subnet usually does not have a route to the Internet Gateway.

Example:

Destination Target
VPC CIDR Local

Notice:

0.0.0.0/0 → IGW
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does not exist.

Result:

  • ❌ No direct internet access
  • ❌ Internet users cannot reach resources
  • ✅ Better security

Real-World Architecture Example

Let's build a simple e-commerce website.

Public Layer

  • Load Balancer
  • Web Server

Private Layer

  • Database

Architecture:

Internet
    ↓
Internet Gateway
    ↓
Public Subnet
    ↓
Load Balancer
    ↓
Web Server
    ↓
Private Subnet
    ↓
Database
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Why Keep the Database in a Private Subnet?

Imagine if the database was directly accessible from the internet.

Anyone could attempt:

  • Brute-force attacks
  • Unauthorized access
  • Data theft

Instead, we design the architecture like this:

Internet
    ↓
Web Server
    ↓
Database
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Only the Web Server can communicate with the Database.

Users cannot directly reach it.

This is a core AWS security principle.

Public vs Private Subnet Visualization 

                    Internet
                        │
                Internet Gateway
                        │
                 Route Table
                        │
          ┌─────────────┴─────────────┐
          │                           │
          ▼                           ▼

    Public Subnet              Private Subnet

          │                           │
          ▼                           ▼

      Web Server                 Database
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Conclusion

Internet Gateway and Route Tables are the foundation of AWS networking.

In this article, we learned:

  • What an Internet Gateway is
  • Why it is required
  • What Route Tables are
  • How AWS routes traffic
  • Difference between Public and Private Subnets
  • Real-world examples of traffic flow
  • How web applications securely communicate with databases

Once you understand Internet Gateway and Route Tables, AWS networking becomes much easier to visualize.

Key Interview Question

What makes an EC2 instance publicly accessible?

The following four conditions must be met:

0.0.0.0/0 Route
        +
Internet Gateway
        +
Public IP
        +
Security Group Allow Rule
        =
Accessible from Internet
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Remembering these four requirements helps answer many AWS networking interview questions.

Next Article

In the next article, we'll explore:

  • Security Groups
  • Network ACLs (NACLs)
  • How AWS protects resources at the network level

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