Layer 3: Network Layer (OSI Model)

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📘 Overview

The Network Layer is Layer 3 of the OSI Model.
It handles the logical addressing and routing of data between devices across multiple networks — even across the globe.

Think of it as:
“How does data travel from Seattle to New York or from your laptop to Google servers?”

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🔧 Key Responsibilities

• Logical Addressing using IP Address
• Routing data between networks
• Path selection (best route)
• Packet forwarding
• Fragmentation of large packets
• Error reporting using ICMP

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📦 Data Unit: Packet

Layer 3 works with Packets — containers that include:

• Source IP Address
• Destination IP Address
• Data payload from higher layers

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🛠 Devices Operating at Layer 3

Device	Function
Router	Directs packets between networks
Layer 3 Switch	Combines switch speed with routing capability
Firewalls	Often inspect Layer 3 & above

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🔁 Protocols at Layer 3

Protocol	Purpose
IP	Logical addressing (IPv4/IPv6)
ICMP	Diagnostics (Ping, Traceroute)
ARP	Resolves IP to MAC addresses
RIP, OSPF, BGP	Routing protocols

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🧪 Troubleshooting at Layer 3

• Use ping to test reachability
• Use tracert (Windows) or traceroute (Linux/Mac) to trace packet paths
• Check for IP conflicts
• Review router configurations
• Check subnetting issues

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🧠 Real-Life Scenario

When you visit www.google.com:

• Layer 3 breaks your data into packets.
• Each packet finds its own path across the internet.
• Routers read the destination IP address and forward it along the best route.

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📊 Visual Aids

• IP Packet Diagram
• Router Path Diagram

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🔁 Related Pages

• IP Address
• Router
• Subnetting
• Layer 2: Data Link Layer
• Layer 4: Transport Layer
• ICMP
• ARP

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Return to: OSI Model Overview