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• 🛣️ Route Tables – Complete Explanation
  • 1️⃣ What is a Route Table?
  • 2️⃣ Why Route Tables Are Needed
  • 3️⃣ Basic Route Table Concepts
  • 4️⃣ How Route Tables Work (Simple Logic)
  • 5️⃣ Example of a Simple Route Table
    • What this means:
  • 6️⃣ Default Route (0.0.0.0/0)
  • 7️⃣ Longest Prefix Match (Very Important)
  • 8️⃣ Route Tables in AWS (VPC Context)
  • 9️⃣ Default Route Table in AWS
  • 🔟 Common Route Table Targets in AWS
  • 1️⃣1️⃣ Public Subnet Route Table
    • Route Table:
    • Result:
  • 1️⃣2️⃣ Private Subnet Route Table
    • Route Table:
    • Result:
  • 1️⃣3️⃣ Route Table Association
  • 1️⃣4️⃣ What Route Tables Do NOT Do
  • 1️⃣5️⃣ Route Tables vs Firewalls
  • 1️⃣6️⃣ Real AWS Traffic Flow Example
    • Private EC2 → Internet
  • 1️⃣7️⃣ Common AWS Exam Traps
  • 1️⃣8️⃣ Route Table for VPC Peering
  • 1️⃣9️⃣ Route Propagation
  • 2️⃣0️⃣ One-Line Exam Definition
  • 2️⃣1️⃣ Simple Analogy
  • 2️⃣2️⃣ Final Summary

🛣️ Route Tables – Complete Explanation

1️⃣ What is a Route Table?

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

Where to send traffic next.

In simple words:

A route table decides the path (route) that network packets should take.

2️⃣ Why Route Tables Are Needed

When a packet reaches a router or subnet, the system must answer:

“Where should this packet go?”

Without route tables:

• Packets would be lost
• Networks wouldn’t know how to communicate

3️⃣ Basic Route Table Concepts

A route table contains multiple routes.

Each route has:

4️⃣ How Route Tables Work (Simple Logic)

1. Packet arrives
2. Destination IP is checked
3. Route table is scanned
4. Most specific route (longest prefix) is chosen
5. Packet is forwarded to target

5️⃣ Example of a Simple Route Table

What this means:

• Traffic within 10.0.0.0/16 stays inside VPC
• All other traffic goes to the internet

6️⃣ Default Route (0.0.0.0/0)

0.0.0.0/0 means:

Any destination not matched by other routes

This is called the default route.

7️⃣ Longest Prefix Match (Very Important)

If multiple routes match:

• The route with more specific CIDR wins

Example:

10.0.1.0/24 → local
10.0.0.0/16 → IGW

Traffic to 10.0.1.5 uses /24, not /16.

8️⃣ Route Tables in AWS (VPC Context)

In AWS:

• Every VPC has route tables
• Route tables are associated with subnets
• One route table per subnet

9️⃣ Default Route Table in AWS

When you create a VPC:

• AWS creates a main route table
• All subnets use it unless changed

Default route:

VPC CIDR → local

🔟 Common Route Table Targets in AWS

1️⃣1️⃣ Public Subnet Route Table

Route Table:

Result:

• EC2 gets public internet access (if public IP exists)

1️⃣2️⃣ Private Subnet Route Table

Route Table:

Result:

• Outbound internet allowed
• No inbound internet access

1️⃣3️⃣ Route Table Association

• Each subnet:

  • Must be associated with one route table

• Route table:

  • Can be associated with multiple subnets

1️⃣4️⃣ What Route Tables Do NOT Do

❌ They do NOT allow or block traffic ❌ They do NOT act as firewalls

👉 That’s the job of:

• Security Groups
• NACLs

1️⃣5️⃣ Route Tables vs Firewalls

1️⃣6️⃣ Real AWS Traffic Flow Example

Private EC2 → Internet

1. EC2 sends packet
2. Route table sees 0.0.0.0/0
3. Sends traffic to NAT Gateway
4. NAT sends to IGW
5. Internet receives traffic

1️⃣7️⃣ Common AWS Exam Traps

❗ EC2 has public IP but no internet?

• Check route table

❗ Private EC2 reachable from internet?

• Impossible without IGW + public subnet

❗ VPC peering not working?

• Missing route entry

1️⃣8️⃣ Route Table for VPC Peering

Example:

VPC A CIDR → Peering Connection

Both VPCs must have:

• Correct route entries

1️⃣9️⃣ Route Propagation

Some gateways (VPN, TGW) can:

• Automatically add routes
• Using route propagation

2️⃣0️⃣ One-Line Exam Definition

A route table contains rules that determine where network traffic is directed within and outside a VPC.

2️⃣1️⃣ Simple Analogy

Route Table = Google Maps Firewall = Police Checkpoint

2️⃣2️⃣ Final Summary

• Route tables decide traffic paths
• They work on destination IP
• Public vs Private subnets depend on route tables
• They don’t provide security filtering
• Essential for AWS networking