Why We Need the Network Layer

Why We Have So Far

Inside one building:

• devices communicate using Ethernet or Wi-Fi
• switches deliver frames efficiently
• MAC addresses identify devices
• errors are detected and controlled

Example:

Laptop -> Switch -> Printer

Everything works beautifully.

But now comes a bigger challenge.

The Big Question

Imagine this situation:

You open a website:

www.example.com

Your request travels to a server possibly thousands of kilometers away.

But wait…

Your LAN only knows about local devices.

So how does your computer reach:

• another city?
• another ISP?
• another country?

Layer 2 cannot solve this.

The Limitation of MAC Addresses

MAC addresses work only locally.

Why?

Because switches operate inside a single network.

Example MAC:

00:1A:2B:3C:4D:5E

Problems if we used MAC globally:

Problem 1: No Hierarchy

MAC addresses are flat identifiers.

They do not indicate location.

Switches would need to know every device on Earth.

Impossible.

Problem 2: Massive Broadcasts

Frames would flood globally.

Internet traffic would collapse instantly.

Problem 3: Scalability Failure

Billions of devices exist.

Layer 2 networks cannot scale that far.

So networking needed a new idea:

A system that allows networks to connect to other networks.

The Key Insight – Network of Networks

Instead of one giant LAN:

We connect many small networks together.

Home Network → ISP → Regional Network → Global Internet → Server Network

This is literally why it is called:

Inter-net (Interconnected networks)

To achieve this, we needed:

• global addressing
• path selection
• forwarding between networks

This is the job of the Network Layer.

What Is the Network Layer?

The Network Layer (Layer 3) is responsible for:

Delivering data from one network to another across the world.

It introduces two revolutionary concepts:

1. IP Addressing – global identity
2. Routing – finding paths between networks

Introducing IP Addresses

Instead of hardware identifiers (MAC):

Devices now get logical addresses.

Example:

192.168.1.10

This is an IP address.

Unlike MAC addresses:

• hierarchical
• location-aware
• scalable worldwide

Analogy: Postal System

MAC Address = Person's fingerprint

IP Address = Postal address

A postal address tells:

• country
• city
• street
• house

Routing becomes possible.

Routers – The New Network Devices

Switches deliver frames locally.

Routers deliver packets between networks.

Example:

Laptop → Switch → Router → Internet → Router → Server

Router acts like a border checkpoint between networks.

Its job:

Decide where data should go next.

Packets Instead of Frames

Layer 2 uses frames.

Layer 3 introduces:

Packets

Structure:

| IP Header | Data |

Packets can travel across many networks.

Each router forwards them step by step.

Routing – Finding the Path

Imagine traveling from Delhi to New York.

You don't jump directly.

You travel:

Delhi → Dubai → London → New York

Routers do the same.

Each router decides:

“Where should this packet go next?”

This process is called routing.

Hop-by-Hop Delivery

Packets move one step at a time.

Each movement between routers is called a:

Hop

Example:

Your PC → Router1 → Router2 → Router3 → Server

Each router only knows the next best step.

No device knows the entire path beforehand.

Why Layer 3 Changed Everything

Layer 3 enabled:

• global communication
• scalable internet growth
• independent networks connecting freely
• worldwide data delivery

Without Layer 3:

There would be no internet – only isolated LANs.

Layer 2 vs Layer 3

Real Example – Opening a Website

When you open a website:

1. Laptop creates IP packet.
2. Sends to local router.
3. Router forwards to ISP.
4. Multiple routers forward packet.
5. Servers receives request.

All powered by the Network Layer.

Big Picture So Far

Your networking knowledge now looks like:

Layer 3 — Network Layer (Global delivery)
Layer 2 — Data Link Layer (Local delivery)
Layer 1 — Physical Layer (Signals)

We have moved from electricity -> local networks -> global communication.