Computer Networks
CN Part-1
- What is a Computer Network?
- Goals and Applications of Computer Networking
- Computer Network Components
- Types of Computer Networks
- Network Topology
- Difference between Client Server and Peer to Peer Network
- Layered Architecture in Computer Networks
- Protocol Hierarchy in Computer Networks
- Interfaces and Services in Computer Networks
- Connection Oriented and Connectionless Services
- Service Primitives
- OSI Model
- TCP/IP Model
- Difference between OSI Model and TCP/IP Model
- Encapsulation and Decapsulation in Computer Networks
- Queueing Models in Computer Network
CN Part-2
- Physical Layer in Computer Networks
- Transmission Media in Computer Networks
- Analog and Digital Signals
- Data Rate and Bandwidth
- Encoding Techniques
- Multiplexing in Computer Networks
- Transmission Modes in Computer Networks
- Data Link Layer
- Framing in Data Link Layer
- Flow Control in Data Link Layer
- Error Control in Data Link Layer
- Error Detection and Correction in Computer Networks
- Stop and Wait Protocol
- Sliding Window Protocol
- SDLC(Synchronous Data Link Control) Protocol
- High-Level Data Link Control (HDLC) Protocol
- Point-to-Point Protocol (PPP)
CN Part-3
- MAC Sublayer and MAC Addressing
- Channel Allocation Methods
- ALOHA Protocol (Pure and Slotted ALOHA)
- CSMA/CD and CSMA/CA in Computer Networks
- Ethernet (IEEE 802.3)
- Wireless LANs (IEEE 802.11, Wi-Fi, and Bluetooth Basics)
- Network Layer and Design Issues
- IPv4 and IPv6 Protocols
- Addressing: IP Address, Subnetting, and Supernetting
- Routing Algorithms: Static vs Dynamic, Distance Vector, and Link State
- Routing Protocols: RIP, OSPF, and BGP
- Packet Switching and IP Fragmentation
- Network Layer Protocols: ICMP, ARP, RARP, DHCP, and NAT
CN Part-4
- Transport Layer and Functions
- Process-to-Process Delivery, Ports, and Sockets
- TCP Segment and Three-Way Handshake Connection Establishment
- UDP (User Datagram Protocol): Format and Features
- Flow Control and Error Control in Transport Layer
- Congestion Control: Leaky Bucket, Token Bucket, and TCP Congestion Control
CN Part-5
- Application Layer Architecture and Principles
- Domain Name System (DNS)
- Email Protocols: SMTP, POP3, and IMAP
- File and Web Protocols: FTP, HTTP, and HTTPS
- Remote Access Protocols: Telnet and SSH
- World Wide Web (WWW) and Web Architecture
- Network Security Basics: SSL, TLS, and Cryptography Fundamentals
Email Protocols: SMTP, POP3, and IMAP in Computer Networks
Email Architecture Overview
Electronic Mail (Email) internet ki sabse puraani aur widely used applications mein se ek hai.
Email system do distinct stages mein kaam karta hai:
- Email Sending / Pushing: Sender ke client se sender mail server tak, aur fir destination mail server tak message bhejna (Push Protocol).
- Email Retrieving / Pulling: Receiver dwara apne mail server ke mailbox se emails apne device par download ya view karna (Pull Protocol).
Is do-tarfa architecture ki wajah se email sending aur receiving ke liye alag-alag protocols use hote hain: Sending ke liye SMTP, aur Receiving ke liye POP3 ya IMAP.
[ Alice (Sender) ]
|
|===== 1. SMTP (Push) =====> [ Alice's Mail Server ]
|
|===== 2. SMTP (Push) =====> [ Bob's Mail Server ]
|
[ Bob (Receiver) ] <===== 3. POP3 or IMAP (Pull) ==================================|
1. SMTP (Simple Mail Transfer Protocol)
SMTP ek standard Push Protocol hai jo emails transfer karne ke liye use hota hai (RFC 5321).
Yeh reliable transport ke liye TCP use karta hai.
Port Numbers:
- Port 25: Original server-to-server relay port.
- Port 587: Modern standard client submission port with STARTTLS encryption.
- Port 465: SMTPS (SMTP over SSL/TLS).
Limitation & MIME: Original SMTP sirf 7-bit ASCII text carry kar sakta tha (images, videos ya non-English text nahi bhej sakta tha). Is kami ko door karne ke liye MIME (Multipurpose Internet Mail Extensions) introduce kiya gaya jo binary files aur attachments ko text mein encode karke SMTP se bhejta hai.
2. POP3 (Post Office Protocol Version 3)
POP3 ek simple Message Access / Pull Protocol hai (RFC 1939).
Working: "Download and Delete" mode:
Jab client POP3 se connect hota hai, toh server se saare emails client ke local computer par download ho jaate hain aur server se permanently delete kar diye jaate hain.Ports: Plaintext: Port 110 | Secure (SSL/TLS): Port 995.
Drawback: Multi-device sync impossible hai. Agar aapne laptop par email download kar liya, toh phone par wo email nahi dikhega kyunki server se delete ho chuka hai!
3. IMAP (Internet Message Access Protocol)
IMAP (IMAP4) modern aur powerful Message Access Protocol hai jo multi-device world ke liye design kiya gaya hai (RFC 3501).
Working: Server-Centric (Syncing) mode:
Emails hamesha central Mail Server par hi store rehte hain. Client sirf unka view aur headers dekhta hai. Jab aap phone par email read, delete, ya move karte hain, toh wo changes server par sync ho jaate hain aur turant laptop aur tablet par bhi update ho jaate hain!Ports: Plaintext: Port 143 | Secure (SSL/TLS): Port 993.
Features: Server-side folders, partial message fetch (sirf text download karna bina bade attachments ke), multi-device real-time synchronization.
POP3 vs IMAP Comparison (Exam Favorite!)
| Feature | POP3 | IMAP (IMAP4) |
|---|---|---|
| Storage Location | Emails download hokar Local Device par store hote hain. | Emails hamesha Central Mail Server par store rehte hain. |
| Server Deletion | Download ke baad server se delete ho jaate hain (default). | Server par retained rehte hain jab tak user manually delete na kare. |
| Multiple Device Sync | Not supported (Single device access). | Fully supported (Phone, laptop, browser fully synchronized). |
| Bandwidth & Server Load | Server par zero storage load, offline reading easy. | Server storage require karta hai, internet active chahiye. |
| Default Ports | Port 110 (Insecure), Port 995 (Secure). | Port 143 (Insecure), Port 993 (Secure). |
Summary Table
| Protocol | Type | Role |
|---|---|---|
| SMTP | Push Protocol | Emails ko send aur mail servers ke beech transfer karna (Port 25 / 587). |
| POP3 | Pull Protocol | Server se device par email download karke delete karna (Port 110 / 995). |
| IMAP | Sync / Pull Protocol | Server par emails rakh kar multi-device real-time synchronization (Port 143 / 993). |