Day 1: Network Fundamentals & the OSI Model

What You'll Learn Today

What a computer network is and why networks exist
Types of networks: PAN, LAN, MAN, and WAN
The OSI 7-layer model and the role of each layer
How data encapsulation works across layers
How the TCP/IP model compares to the OSI model

What Is a Computer Network?

A computer network is a collection of interconnected devices that can exchange data. The devices — computers, servers, phones, printers, IoT sensors — communicate over wired or wireless media using agreed-upon rules called protocols.

Networks exist to share resources (files, printers, internet access), enable communication (email, video calls), and provide centralized management (authentication, backups).

flowchart LR
    subgraph Network["Simple Computer Network"]
        A["💻 Laptop"]
        B["🖥️ Server"]
        C["🖨️ Printer"]
        D["📱 Phone"]
    end
    A --- B
    A --- C
    B --- D
    style Network fill:#3b82f6,color:#fff

At its core, every network relies on three elements:

Element	Description	Example
Nodes	Devices that send or receive data	Computers, routers, switches
Links	Physical or wireless connections between nodes	Ethernet cables, Wi-Fi signals
Protocols	Rules governing how data is formatted and transmitted	HTTP, TCP, IP

Types of Networks

Networks are classified primarily by their geographic scope.

flowchart LR
    subgraph PAN["PAN\n< 10 m"]
        P1["Bluetooth\nheadset"]
    end
    subgraph LAN["LAN\n< 1 km"]
        L1["Office\nnetwork"]
    end
    subgraph MAN["MAN\n< 50 km"]
        M1["City-wide\nnetwork"]
    end
    subgraph WAN["WAN\nGlobal"]
        W1["The\nInternet"]
    end
    PAN --> LAN --> MAN --> WAN
    style PAN fill:#8b5cf6,color:#fff
    style LAN fill:#3b82f6,color:#fff
    style MAN fill:#22c55e,color:#fff
    style WAN fill:#f59e0b,color:#fff

Type	Full Name	Range	Speed	Example
PAN	Personal Area Network	~10 meters	Low–Medium	Bluetooth devices, USB tethering
LAN	Local Area Network	Up to ~1 km	High (1–10 Gbps)	Office or home network
MAN	Metropolitan Area Network	Up to ~50 km	Medium–High	City-wide cable TV network
WAN	Wide Area Network	Unlimited	Varies	The Internet, corporate MPLS

PAN (Personal Area Network)

A PAN connects devices within your immediate personal space. Bluetooth pairing your phone to headphones creates a PAN. These networks are small, low-power, and short-range.

LAN (Local Area Network)

A LAN covers a building or campus. Most office networks and home Wi-Fi setups are LANs. They use Ethernet (IEEE 802.3) or Wi-Fi (IEEE 802.11) and offer high bandwidth with low latency.

MAN (Metropolitan Area Network)

A MAN spans a city or large campus. Internet service providers often operate MANs to connect multiple LANs across a metropolitan area. Cable TV networks are a classic example.

WAN (Wide Area Network)

A WAN connects LANs across cities, countries, or continents. The Internet is the largest WAN. Organizations also build private WANs using leased lines or MPLS to connect branch offices.

The OSI 7-Layer Model

The Open Systems Interconnection (OSI) model, published by ISO in 1984, is a conceptual framework that standardizes network communication into seven layers. Each layer has a specific responsibility and communicates with the layers directly above and below it.

flowchart TB
    subgraph OSI["OSI Model"]
        L7["Layer 7 — Application\nHTTP, FTP, SMTP, DNS"]
        L6["Layer 6 — Presentation\nSSL/TLS, JPEG, ASCII"]
        L5["Layer 5 — Session\nNetBIOS, RPC"]
        L4["Layer 4 — Transport\nTCP, UDP"]
        L3["Layer 3 — Network\nIP, ICMP, OSPF"]
        L2["Layer 2 — Data Link\nEthernet, Wi-Fi, ARP"]
        L1["Layer 1 — Physical\nCables, Hubs, Signals"]
    end
    L7 --> L6 --> L5 --> L4 --> L3 --> L2 --> L1
    style L7 fill:#ef4444,color:#fff
    style L6 fill:#f59e0b,color:#fff
    style L5 fill:#f59e0b,color:#fff
    style L4 fill:#22c55e,color:#fff
    style L3 fill:#3b82f6,color:#fff
    style L2 fill:#8b5cf6,color:#fff
    style L1 fill:#8b5cf6,color:#fff

Layer 1: Physical

The Physical layer deals with the raw transmission of bits over a physical medium. It defines voltages, pin layouts, cable specifications, and signal timing.

Devices: Hubs, repeaters, cables, connectors
Examples: Cat5e/Cat6 Ethernet cables, fiber optic, radio waves (Wi-Fi)
PDU: Bits

Layer 2: Data Link

The Data Link layer provides node-to-node data transfer within the same network segment. It handles framing, MAC addressing, and error detection.

Devices: Switches, bridges, NICs
Protocols: Ethernet (IEEE 802.3), Wi-Fi (IEEE 802.11), PPP
PDU: Frames

Layer 3: Network

The Network layer handles logical addressing and routing — moving packets from source to destination across multiple networks.

Devices: Routers, Layer 3 switches
Protocols: IP, ICMP, OSPF, BGP
PDU: Packets

Layer 4: Transport

The Transport layer provides end-to-end communication between applications. It manages segmentation, flow control, error recovery, and port numbers.

Protocols: TCP (reliable), UDP (fast, connectionless)
PDU: Segments (TCP) / Datagrams (UDP)

Layer 5: Session

The Session layer manages sessions — ongoing exchanges between two nodes. It handles setup, maintenance, and teardown of connections.

Protocols: NetBIOS, RPC, PPTP
PDU: Data

Layer 6: Presentation

The Presentation layer handles data translation, encryption, and compression. It ensures the application layer receives data in a usable format.

Functions: Encryption (SSL/TLS), data formatting (JPEG, ASCII, MPEG)
PDU: Data

Layer 7: Application

The Application layer is the interface between the network and the end user's software. It provides network services directly to applications.

Protocols: HTTP, FTP, SMTP, DNS, SSH
PDU: Data

Layer	Name	Key Function	Devices/Protocols	PDU
7	Application	User interface to network	HTTP, DNS, FTP	Data
6	Presentation	Data format, encryption	SSL/TLS, JPEG	Data
5	Session	Session management	NetBIOS, RPC	Data
4	Transport	End-to-end delivery	TCP, UDP	Segment/Datagram
3	Network	Routing, logical addressing	IP, ICMP, OSPF	Packet
2	Data Link	Framing, MAC addressing	Ethernet, Wi-Fi	Frame
1	Physical	Bit transmission	Cables, hubs	Bits

Mnemonic: "All People Seem To Need Data Processing" (top-down) or "Please Do Not Throw Sausage Pizza Away" (bottom-up).

Data Encapsulation

When data travels from an application down through the OSI layers, each layer wraps the data with its own header (and sometimes a trailer). This process is called encapsulation. The receiving side reverses the process — de-encapsulation.

flowchart TB
    subgraph Encapsulation["Data Encapsulation"]
        D["Application Data"]
        S["Transport Header + Data = Segment"]
        P["Network Header + Segment = Packet"]
        F["Data Link Header + Packet + Trailer = Frame"]
        B["Physical: Bits on the wire"]
    end
    D --> S --> P --> F --> B
    style D fill:#ef4444,color:#fff
    style S fill:#22c55e,color:#fff
    style P fill:#3b82f6,color:#fff
    style F fill:#8b5cf6,color:#fff
    style B fill:#f59e0b,color:#fff

Layer	PDU Name	What Is Added
Application / Presentation / Session	Data	Application-level formatting
Transport	Segment (TCP) or Datagram (UDP)	Source/destination port, sequence numbers
Network	Packet	Source/destination IP address, TTL
Data Link	Frame	Source/destination MAC address, FCS (error check)
Physical	Bits	Electrical/optical signals

Each layer only communicates with its peer layer on the remote device. A router reading a packet does not care about the application data — it reads the Layer 3 header to make routing decisions, then forwards the packet.

TCP/IP Model vs. OSI Model

The TCP/IP model (also called the Internet model) is the practical model that the Internet actually uses. It has four layers that map to the OSI model.

flowchart TB
    subgraph OSI["OSI Model (7 Layers)"]
        O7["Application"]
        O6["Presentation"]
        O5["Session"]
        O4["Transport"]
        O3["Network"]
        O2["Data Link"]
        O1["Physical"]
    end
    subgraph TCPIP["TCP/IP Model (4 Layers)"]
        T4["Application"]
        T3["Transport"]
        T2["Internet"]
        T1["Network Access"]
    end
    O7 -.-> T4
    O6 -.-> T4
    O5 -.-> T4
    O4 -.-> T3
    O3 -.-> T2
    O2 -.-> T1
    O1 -.-> T1
    style OSI fill:#3b82f6,color:#fff
    style TCPIP fill:#22c55e,color:#fff

OSI Layer(s)	TCP/IP Layer	Protocols
7, 6, 5 — Application, Presentation, Session	Application	HTTP, FTP, DNS, SMTP, SSH
4 — Transport	Transport	TCP, UDP
3 — Network	Internet	IP, ICMP, ARP
2, 1 — Data Link, Physical	Network Access	Ethernet, Wi-Fi, PPP

Key differences:

The OSI model is a theoretical reference; TCP/IP is a practical implementation.
OSI has 7 layers; TCP/IP consolidates them into 4.
OSI separates Presentation and Session; TCP/IP rolls them into the Application layer.
Most real-world troubleshooting uses the TCP/IP model, but the OSI model is invaluable for understanding concepts and certification exams.

Summary

Summary Table

Concept	Key Point
Computer network	Interconnected devices sharing resources via protocols
Network types	PAN < LAN < MAN < WAN by geographic scope
OSI model	7-layer conceptual framework for network communication
Encapsulation	Each layer wraps data with its own header/trailer
TCP/IP model	4-layer practical model that powers the Internet
PDU	Data unit changes name at each layer (data → segment → packet → frame → bits)

Key Takeaways

Networks exist to share resources and enable communication between devices.
The OSI model divides networking into 7 layers, each with a clear responsibility.
Data encapsulation adds headers at each layer as data moves down the stack.
The TCP/IP model is the practical counterpart to the OSI model, using 4 layers.
Understanding layered models is essential for diagnosing where network problems occur.

Practice Problems

Beginner

Name the 7 layers of the OSI model from bottom to top. For each layer, give one example protocol or device.
A user reports they cannot load a website. At which OSI layer does HTTP operate? At which layer would you check if the Ethernet cable is plugged in?
What is the PDU name at each of these layers: Transport, Network, Data Link?

Intermediate

Explain the difference between a hub (Layer 1) and a switch (Layer 2). Why does a switch improve network performance?
A packet travels from your laptop to a web server across the Internet. Describe the encapsulation process from the Application layer down to the Physical layer.
Compare the OSI and TCP/IP models. Why does the TCP/IP model combine Layers 5, 6, and 7 into a single Application layer?

Advanced

You run ping 8.8.8.8 and it succeeds, but ping google.com fails. Using the OSI model, identify which layer is likely causing the problem and explain why.
A company has offices in Tokyo and New York connected by a leased line. Classify the network within each office and the connection between them by network type (LAN, WAN, etc.). Describe which OSI layers are most relevant at each segment.
Design a diagram showing how a web request (HTTP) gets encapsulated at each OSI layer, transmitted across two routers, and de-encapsulated at the destination server. Identify what each intermediate router examines.

References

Kurose, J. & Ross, K. — Computer Networking: A Top-Down Approach, 8th Edition
Tanenbaum, A. & Wetherall, D. — Computer Networks, 6th Edition
ISO 7498-1 — OSI Basic Reference Model
RFC 1122 — Requirements for Internet Hosts

Next Up

In Day 2, we dive into LAN & WAN technologies — Ethernet, MAC addresses, switches vs. hubs, VLANs, and WAN technologies like MPLS. You will learn how data actually moves within and between networks.