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Routing Architecture in Cloud Computing

Learn about Routing Architecture in this comprehensive Cloud Computing tutorial. Best practices for VPC routing.

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1Custom vs Main Tables

Never use the main route table for public subnets. Keep the main table completely private and explicitely associate custom tables to public subnets. This acts as a fail-safe so any newly created subnet that defaults to the main table remains isolated from the internet.

2Step-by-Step Breakdown

The IGW. An IGW is a highly available VPC component that enables communication with the internet.

Attaching IGW. An IGW must be explicitly attached to your custom VPC.

Main Route Table. Every VPC comes with a main route table containing a 'local' route by default.

Custom Route Tables. Always create custom route tables for public and private subnets instead of modifying the main route table.

Knowledge Check. What route must be added to a route table to make its associated subnets public?

→10.0.0.0/16 -> IGW
→0.0.0.0/0 -> IGW

Adding the Default Route. Add the 0.0.0.0/0 route pointing to the IGW.

Subnet Association. A route table must be explicitly associated with a subnet to govern its traffic.

Route Evaluation. The most specific route (longest prefix match) is always evaluated first. Local routes take precedence over 0.0.0.0/0.

Security Groups vs Route Tables. Route tables determine WHERE traffic goes. Security Groups determine IF traffic is allowed.

Summary. IGWs and custom route tables form the backbone of your public subnet architecture.

Pascual Vila

Frontend Instructor // Code Syllabus

Common Pitfalls & Errors

The Error //

Attaching an Internet Gateway but forgetting to update the route table

aws ec2 create-route --route-table-id rtb-12345678 \
  --destination-cidr-block 0.0.0.0/0 --gateway-id igw-12345678

The Solution //

Attaching an IGW to a VPC does nothing by itself — a subnet is only 'public' if its route table has an explicit route sending 0.0.0.0/0 traffic to that IGW. Without that route, instances in the subnet still can't reach the internet even with a public IP assigned.

The Error //

Assuming a public IP alone makes an instance reachable from the internet

// Checklist for internet reachability:
// 1. Instance has a public/Elastic IP
// 2. Subnet route table has a 0.0.0.0/0 -> igw-xxx route
// 3. Security group allows the inbound port

The Solution //

A public IP is necessary but not sufficient — the subnet's route table must route to an IGW, and the instance's security group must allow the relevant inbound port. Missing any one of these three pieces looks identical from the outside: the instance is simply unreachable.