Networking Terms

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Networking Basics: History, OSI Model, and TCP/IP Model

🌐 History & Models

  • 1970s Introduction (OSI Model)

    • IBM started with the idea of making networking standardized for all its departments.

    • Later, ISO (International Organization for Standardization) created the OSI model (Open Systems Interconnection) as a universal reference model for how data should move through a network.

  • Department of Defense (DoD) – TCP/IP Model

    • In the U.S., the DoD created their own practical networking model, which became the TCP/IP model.

    • It was first standardized in the 1990s.

    • By the late 2000s, TCP/IP was universally adopted by vendors (Cisco, Microsoft, etc.).

    • Today, TCP/IP is the real-world standard — OSI is more for learning and reference.


🏗️ OSI Model (7 Layers)

Think of OSI like a building with 7 floors, each handling a specific function.

  1. Application Layer

    • Closest to the user.

    • Examples: FTP, HTTP, SMTP, DNS

    • Purpose: Provides network services directly to applications (like browsers, email clients).

  2. Presentation Layer

    • Formats or translates data.

    • Example: Converts a file into .jpg, .mp3, .pdf, encryption (SSL/TLS).

    • Purpose: Makes sure data is readable by the receiving system.

  3. Session Layer

    • Manages the “conversation” between two devices.

    • Example: Keeps track of your login session to a website.

    • Purpose: Establish, maintain, and end communication.

  4. Transport Layer

    • Ensures reliable delivery of data.

    • Protocols: TCP (reliable, connection-based), UDP (fast, no guarantee).

    • Example: Web browsing (TCP), video streaming (UDP).

  5. Network Layer

    • Deals with IP addresses and routing.

    • Protocols: IP, ICMP (ping), ARP.

    • Purpose: Finds the best path to deliver packets across networks.

  6. Data Link Layer

    • Works with MAC addresses and ensures data moves from one device to another on the same network.

    • Protocols: Ethernet, PPP, Switches.

    • Example: Your laptop talking to the Wi-Fi router using Ethernet frames.

  7. Physical Layer

    • The actual hardware and media.

    • Examples: Cables, Wi-Fi signals, voltages, fiber optics.

    • Purpose: Converts binary (1s & 0s) into electrical/light/radio signals.


📦 Data Flow

  • Encapsulation (top → bottom)
    Data starts at the Application layer (user sending a message), and each lower layer wraps it with its own header info until it becomes bits on the wire.

  • Decapsulation (bottom → top)
    On the receiving side, data travels upward, each layer removing its wrapper until the Application layer gets the original message.


⚡ TCP/IP Model (Real-World Model)

The TCP/IP model is simpler — it has 4 layers, but maps to OSI.

  1. Application Layer

    • Combines OSI’s Application + Presentation + Session layers.

    • Examples: HTTP, FTP, DNS, Email.

  2. Transport Layer

    • Same as OSI Transport.

    • Examples: TCP, UDP.

  3. Network Layer

    • Same as OSI Network.

    • Examples: IP, ICMP, OSPF.

  4. Network Access Layer (Data Link + Physical)

    • Combines OSI’s Data Link + Physical.

    • Examples: Ethernet, Wi-Fi, cabling.