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OSI Reference Model

OSI Model ko ISO (International Organization for Standardization) ne 1984 me banaya tha.
Yeh ek 7-layer architecture hai jo batata hai ki data network me kaise travel karta hai.

Yaad rakhne ka trick:
A – P – S – T – N – D – P
All People Seem To Need Data Processing

1. Physical Layer (Layer 1)

Yeh hardware wali layer hai.
Isme cables, connectors, signals (voltage, light, radio waves) sab aata hai.
Binary data ko signal me convert karta hai.
Devices: Hub, Repeater, Modem, Cables

Modes of communication:

Simplex
Half-duplex
Full-duplex

2. Data Link Layer (Layer 2)

Same network ke devices ke beech communication handle karta hai.
MAC Address yahi par hota hai.
Frames create karta hai aur error detection (CRC) karta hai.

Do sub-layers:

MAC — physical addressing (MAC address)
LLC — network layer ke liye control

Devices: Switch, Bridge, NIC

3. Network Layer (Layer 3)

IP Addressing aur Routing yahan hota hai.
Data ko ek network se doosre network tak bhejne ka kaam.

Protocols:

Routed: IP, IPX, AppleTalk
Routing: RIP, OSPF, EIGRP, IGRP

Devices: Router, Layer-3 Switch

4. Transport Layer (Layer 4)

Data ko small pieces (segments) me todta hai.
Reliable / unreliable delivery ensure karta hai.
Yahi par Port Numbers use hote hain.

🔹 TCP

Reliable
Slow
Connection-oriented
Protocol No. 6
Example: HTTP, FTP, SMTP

🔹 UDP

Fast
No reliability
Connection-less
Protocol No. 17
Example: DNS, DHCP, TFTP

Port Numbers:

Total: 65535
1–1023 → Well-known
1024–49151 → Registered
49152–65535 → Dynamic

5. Session Layer (Layer 5)

Communication sessions ko create, manage aur close karta hai.
Example: Web server ek time par 1000 users ko handle karta hai.

6. Presentation Layer (Layer 6)

Data ko format aur encode karta hai.
Example: ASCII, Encryption, Compression
Dono systems me data readable banata hai.

7. Application Layer (Layer 7)

Yahi par aapki applications kaam karti hain.
Example: Email (SMTP), Web (HTTP), File Transfer (FTP)

Router Basics (Super Simple)

Router kya hota hai?

Do alag-alag networks ko connect karta hai.
Best path choose karta hai.
Layer 3 device.

Companies: Cisco, Juniper, Huawei (but Cisco best in market).

Router Types

Fixed Router
- Upgradable nahi
- Mostly access layer me use hota hai
Modular Router
- Upgradable
- Distribution & core layer me use hota hai

Important Router Ports

🔹 AUI Port (LAN)

15-pin
Ethernet LAN connect karne ke liye

🔹 Serial Port (WAN)

Remote site connection
V.35 cable used

🔹 Console Port

Router configuration ke liye
RJ-45
Most sensitive port

🔹 AUX Port

Remote administration ke liye

Cisco 3-Layer Network Model

1) Core Layer

Fast switching
High-speed backbone

2) Distribution Layer

Routing between VLANs
Security, ACL, filtering

3) Access Layer

User devices connect
Collision domain break and More.