Routing Basics

💡 Introduction

The internet is an incredible technological achievement that connects millions of networks and enables seamless communication between them. With the ability to access data from anywhere in the world in mere fractions of a second, routing plays a vital role in facilitating this global connectivity.

🌐 Understanding Routing

Routing is the process by which data is forwarded across interconnected networks. In this lesson, you will learn the fundamentals of routing and how routing tables function. Additionally, you will explore major routing protocols, the concept of non-routable address space, and the significance of the RFC (Request for Comments) system in shaping the internet as we know it today. These skills are essential for troubleshooting networking issues, particularly for IT support specialists.

🔁 The Simple and Complex Nature of Routing

At a high level, routing is relatively straightforward. A router, a network device responsible for forwarding traffic based on destination addresses, performs a few basic steps in the routing process. However, beneath the surface, routing is a complex and technologically advanced topic that has been extensively documented in books.

🌍 Routing in Action

Let's take a closer look at how routing works. Imagine a router connected to two networks—Network A with the address space 192.168.1.0/24 and Network B with the address space 10.0.0.0/24. The router has an interface on each network, with IP addresses of 192.168.1.1 and 10.0.0.254, respectively.

When a computer on Network A (e.g., with the IP address 192.168.1.100) sends a packet to the address 10.0.0.10, it recognizes that the destination is not on its local subnets. It sends the packet to the router's interface on Network A, which receives it based on the destination MAC address. The router examines the IP datagram header, determines the destination IP (10.0.0.10), and looks it up in its routing table. Finding that Network B (10.0.0.0/24) is the correct destination network, the router forwards the packet to the interface closest to the remote network, using the MAC address from its ARP table. This process continues until the packet reaches its final destination.

To illustrate a more complex scenario, let's introduce a third network—Network C, with the address space 172.16.1.0/23. Another router connects Network B and Network C, with its interface on Network B having an IP of 10.0.0.1 and its interface on Network C having an IP of 172.16.1.1.

Suppose a computer at 192.168.1.100 wants to send data to a computer with the IP 172.16.1.100. The packet is sent to the router between Network A and Network B (gateway), which inspects the packet, identifies the destination address (172.16.1.100), and consults its routing table. It determines that the quickest way to reach the 172.16.1.0/23 network is through another router with an IP of 10.0.0.1. The packet is then forwarded to the second router, which recognizes that the destination IP is directly connected and forwards the packet to its final destination.

🌐 Routing on a Larger Scale

While our examples involve only a few networks, the internet consists of numerous interconnected routers and networks. Traffic often traverses multiple routers before reaching its intended destination. Additionally, to ensure resilience, core internet routers are typically interconnected in a mesh, providing multiple paths for packets. However, the fundamental concepts of routing—inspecting destination IPs, referring to routing tables, and forwarding packets along the optimal paths—remain the same.

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