📘 PAPER 6 – DATA COMMUNICATION & NETWORKS ( UNIT 5 – APPLICATION LAYER & NETWORK APPLICATIONS )

 

🔴 UNIT 5 – APPLICATION LAYER & NETWORK APPLICATIONS

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🟦 1. Application Layer – Introduction

✅ Definition

The Application Layer is the topmost layer of the OSI model.
It provides services directly to the user.

👉 It acts as an interface between:

• User

• Network

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🟦 2. Functions of Application Layer

✔ File transfer
✔ Email communication
✔ Remote login
✔ Web browsing
✔ Network management

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🟦 3. Application Layer Protocols

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🔹 1. FTP (File Transfer Protocol)

Purpose:

Used to transfer files between client and server.

Features:

✔ Reliable
✔ Uses TCP
✔ Supports upload & download

Ports:

• Port 21 → Control

• Port 20 → Data

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🔹 2. TFTP (Trivial File Transfer Protocol)

Features:

✔ Simpler than FTP
✔ Uses UDP
✔ No authentication
❌ Less secure

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🔹 3. SMTP (Simple Mail Transfer Protocol)

Purpose:

Used to send emails

Characteristics:

✔ Push protocol
✔ Works on TCP
✔ Port 25

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🔹 4. POP3 (Post Office Protocol)

Purpose:

Used to receive emails

Features:

✔ Downloads mail
✔ Deletes mail from server

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🔹 5. IMAP (Internet Message Access Protocol)

Difference from POP:

✔ Emails remain on server
✔ Can access from multiple devices

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🔹 6. HTTP (Hyper Text Transfer Protocol)

Used for:

✔ Web browsing
✔ Communication between browser and server

Features:

✔ Stateless
✔ Uses TCP
✔ Port 80

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🔹 7. HTTPS

Secure version of HTTP

Uses:
✔ SSL / TLS encryption
✔ Port 443

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🟦 4. DNS (Domain Name System)

Definition:

DNS converts:

Domain Name → IP Address

Example:

www.google.com → 142.250.192.14

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DNS Hierarchy:

1. Root Server

2. TLD Server

3. Authoritative Server

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🟦 5. SNMP (Simple Network Management Protocol)

Purpose:

Used to manage and monitor network devices.

Components:

✔ Manager
✔ Agent
✔ MIB

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🟦 6. TELNET

Definition:

Allows remote login to another computer.

❌ Not secure
✔ Replaced by SSH

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🟦 7. Network Applications

Common Applications:

✔ Email
✔ File sharing
✔ Video conferencing
✔ Web browsing
✔ Cloud services

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🟦 8. Client–Server Model

Client:

Requests services

Server:

Provides services

Example:
Browser → Web Server

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🟦 9. Peer-to-Peer Network

Features:

✔ No central server
✔ Each node is client & server
✔ Used in file sharing

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🟦 10. Internet & Public Networks

Internet:

Global network of networks

Public Network:

Accessible by all users

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📌 EXAM IMPORTANT QUESTIONS (UNIT 5)

✔ Explain FTP, SMTP, POP, IMAP
✔ Difference between HTTP & HTTPS
✔ DNS working
✔ Application layer protocols
✔ Client-server vs P2P
✔ SNMP explanation