Unit 1:

Computer Networks

Computer networks connect multiple devices to share resources, exchange data, and communicate. From simple LANs in homes to the global Internet connecting billions of devices, networks form the backbone of modern computing.

What is a Network?

Definition: A computer network is a collection of interconnected devices (computers, servers, printers, mobile phones, IoT devices) that communicate using shared communication protocols and media.

• Resource sharing (files, printers, storage).
• Data exchange and communication.
• Centralized management and security.

Types of Networks

Type	Range / Scope	Example
PAN	1–10 m (personal)	Bluetooth, USB devices.
LAN	Building / campus	Office WiFi, home network.
MAN	City	Cable TV city network.
WAN	Country / global	Internet.

Important Devices

Device	Function
Router	Routing between networks (IP layer).
Switch	MAC‑based forwarding within LAN.
Hub	Simple broadcast on LAN.
Bridge	Connects LAN segments.

Network Topologies

Topology	Structure	Pros / Cons
Bus	Single backbone cable.	Simple & cheap; one fault can break whole network.
Star	All devices → hub/switch.	Easy fault isolation; central point of failure.
Ring	Devices in circle, token‑passing.	Predictable; ring breaks with one link failure.
Mesh	Every device connected to many others.	Very reliable; complex and expensive.

The Internet

The Internet is a global network of networks using the TCP/IP protocol suite. It is decentralized and connects government, academic, corporate, and private networks.

• Packet switching.
• Scalability (handles billions of devices).
• Resilience (no single point of failure).
• Tier‑1, Tier‑2, Tier‑3 ISPs.
• 1983: ARPANET → TCP/IP.
• 1991: WWW (HTTP/HTML).
• 2000s: Broadband explosion.
• 2020s: 5G, IoT explosion.

Protocols

Protocols are standardized rules for communication.

• Syntax: data format (fields, sizes).
• Semantics: meaning of each field.
• Timing: speed, when to send, timeouts.
• Error control: checksums, ACK/NACK.

Protocol	Layer	Purpose
Ethernet	Data Link	LAN communication (MAC addresses).
IP	Network	Logical addressing and routing.
TCP	Transport	Reliable, connection‑oriented delivery.
UDP	Transport	Unreliable, fast datagram transport.
HTTP	Application	Web browsing.

OSI Model (7 Layers)

The OSI model divides network communication into 7 conceptual layers:

1. Physical → bits on wire.
2. Data Link → MAC addresses, frames.
3. Network → IP, routing.
4. Transport → TCP/UDP.
5. Session → connection management.
6. Presentation → encryption, compression.
7. Application → HTTP, DNS, FTP.

Data flow: App Data → Segment → Packet → Frame → Bits (sending).

Receiving: Bits → Frame → Packet → Segment → App Data.

TCP/IP Model (4–5 Layers)

Internet actually uses TCP/IP model:

• Network Access (Physical + Data Link): Ethernet, WiFi.
• Internet (Network): IP, ICMP.
• Transport: TCP, UDP.
• Application (Session+Presentation+Application): HTTP, DNS, FTP, SMTP.

Addressing in Networks

Addresses uniquely identify source and destination.

Address Type	Layer	Scope	Example
MAC (Physical)	Data Link	LAN only	00:1A:2B:3C:4D:5E
IP (Logical)	Network	Global	192.168.1.10
Port	Transport	Process	80 (HTTP)
URL	Application	Resource	google.com/path