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AWS Route Tables Explained: How Traffic Finds Its Destination

Learn how AWS route tables work, understand destination CIDRs, the local route, default routes, and how route tables direct traffic between subnets and the internet.

AWS Route Tables Explained: How Traffic Finds Its Destination

we learned about public and private subnets. But how does AWS decide where network traffic should go?
The answer is route tables.

1. What Is a Route Table?

A route table is a set of rules that determines where network traffic should be directed.
Think of it like a navigation system. When traffic needs to reach a destination, the route table helps determine the next step.
In AWS, route tables control how traffic leaves a subnet or travels to other networks.

2. What Does a Route Look Like?

A route contains two important parts:
  • Destination: The IP address range the traffic is trying to reach.
  • Target: Where AWS should send that traffic.
For example:
DestinationTarget
10.0.0.0/16Local
0.0.0.0/0Internet Gateway
Let's understand each route.

3. Understanding the Local Route

Suppose our VPC uses this CIDR block:
10.0.0.0/16
AWS automatically creates a local route that allows communication between resources within the VPC, subject to security controls and other configuration.
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VPC: 10.0.0.0/16
|
├── Subnet A
|
└── Subnet B
The local route enables traffic between these subnets without requiring an Internet Gateway.
Remember: The local route is automatically created for the VPC's CIDR block.

4. What Is a Default Route?

Consider this route:
0.0.0.0/0 → Internet Gateway
What does it mean?
0.0.0.0/0 represents all IPv4 destinations.
This route tells AWS to send traffic matching that destination range to the specified Internet Gateway, provided no more specific route takes precedence.
A typical public subnet route table might contain:
DestinationTarget
10.0.0.0/16Local
0.0.0.0/0Internet Gateway
For a resource to communicate directly with the internet, it also needs the appropriate IP addressing and security configuration.

5. How Does a Route Table Work?

Imagine an EC2 instance inside a subnet with this route table:
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Destination Target
10.0.0.0/16 local
0.0.0.0/0 igw-example
If the instance sends traffic to another resource inside 10.0.0.0/16, the local route can handle it.
If it sends traffic to an external IPv4 destination, the default route directs it toward the Internet Gateway.
AWS generally selects the most specific matching route. This is called longest-prefix matching.

6. Public vs Private Route Tables

A public subnet's route table commonly contains:
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10.0.0.0/16 → local
0.0.0.0/0 → Internet Gateway
A private subnet's route table might contain:
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10.0.0.0/16 → local
0.0.0.0/0 → NAT Gateway
The second configuration can allow private resources to initiate outbound IPv4 internet connections through a NAT Gateway, while preventing unsolicited internet connections from being initiated directly to those resources.
A private subnet does not have to use a NAT Gateway. Its routes depend on what connectivity the workload requires.

7. Why Are Route Tables Important?

Route tables help us:
  • Control traffic between networks.
  • Configure internet connectivity.
  • Separate public-facing and private resources.
  • Direct traffic through NAT Gateways and other network targets.
  • Build secure, organized AWS architectures.
Incorrect routes can prevent applications from reaching databases, APIs, or external services.

Key Takeaways

  • A route table contains rules for directing network traffic.
  • A destination identifies the network range.
  • A target identifies where matching traffic should go.
  • The local route supports communication within a VPC.
  • 0.0.0.0/0 represents all IPv4 destinations.
  • AWS uses the most specific matching route when selecting a route.

Day 6 Preview

Internet Gateway vs NAT Gateway in AWS
We'll explore the difference between these two components, how they handle internet traffic, and when to use each in a VPC architecture.
#AWS #AWSNetworking #AmazonVPC #CloudComputing #Networking
# aws-networking# networking# routing# amazon-vpc
Any opinions in this article are those of the individual author and may not reflect the opinions of AWS.
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