Everything Routers do - Part 2 - How Routers forward Packets - Networking Fundamentals - Lesson 5

Introduction

This tutorial will guide you through the fundamental processes that routers use to forward packets between hosts in a network. Understanding how routers operate, including their use of ARP tables and routing tables, is essential for anyone interested in networking fundamentals. This tutorial builds on concepts introduced in Part 1 of the series, so it is recommended to familiarize yourself with that content first.

Step 1: Understand the Role of Routers

• Routers are devices that route data between different networks.
• Their primary responsibility is to determine the best path for forwarding packets from the source host to the destination host.

Step 2: Learn About ARP Tables

• ARP (Address Resolution Protocol) tables are used by routers to map IP addresses to MAC addresses.
• Initially, ARP tables start out empty. Routers must populate these tables as they receive packets.

Step 3: Packet Transmission from Host A to Host C

1. Host A sends a packet to Host C:

   • The packet includes the destination IP address of Host C.
   • Host A checks its ARP table to find the MAC address corresponding to Host C’s IP.
   • If the MAC address is not found, Host A will send an ARP request to the network.

2. Router receives the packet:

   • The router examines its routing table to determine the best next hop for the packet.
   • If the destination is not in the router's ARP table, it will also send out an ARP request to learn the MAC address.

3. Router forwards the packet:

   • Once the router has the MAC address, it forwards the packet to the next hop or directly to Host C.

Step 4: Response from Host C to Host A

• Host C processes the packet from Host A:
  • It sends a response back to Host A using the same routing and ARP processes.
  • The response packet travels back through the router which again checks its routing and ARP tables to forward the packet correctly.

Step 5: Application to Internet Routers

• The packet forwarding process described is identical for routers on the Internet.
• Each router along the path uses its routing and ARP tables to ensure packets reach their intended destination efficiently.

Conclusion

Understanding how routers forward packets is critical for grasping networking fundamentals. Key points include the role of ARP tables, the process of packet transmission between hosts, and how these principles apply to routers on the Internet. For further learning, explore additional lessons in the Networking Fundamentals series, focusing on topics such as networking protocols and data movement through the Internet.