Introduction to Computer Networks

A computer network is a system of interconnected computers and devices that can communicate and share resources. It networks can range from small local networks (LANs) to large wide area networks (WANs) like the Internet.

The main objectives are to enable Resource Sharing (using shared printers, storage), Data Communication (emails, messaging), and Connectivity/Collaboration (remote access, shared documents).

Data communication involves the exchange of data between a Sender and a Receiver via a medium. Key components are:
Sender: Device sending data.
Receiver: Device receiving data.
Message: The information being sent.
Protocol: Set of rules governing communication.
Transmission Medium: The path (wired/wireless) for data travel.

A networking model defines the structure and functions of network protocols. The OSI (Open Systems Interconnection) model is a conceptual framework that standardizes the functions of a communication system into seven abstraction layers.

The physical path via which data is transmitted. It can be Guided Media (cables like Ethernet, Fiber Optic) or Unguided Media (wireless signals like Wi-Fi, Microwaves).

A Switch is a networking device that connects multiple devices within a LAN. It uses MAC addresses to forward data packets specifically to the destination device.

A Router interconnects different networks and directs data packets between them using IP addresses. It determines the best path for data to travel.

In Bus Topology, all devices connect to a single central cable called the bus. If the main cable fails, the entire network fails.

In Star Topology, all devices connect to a central hub/switch. It is easy to troubleshoot, but if the central hub fails, the network goes down.

In Ring Topology, devices connect in a closed loop. Data travels in one direction. A failure in one device can disrupt the entire network.

In Mesh Topology, every device connects to every other device. This provides high redundancy and reliability; if one link fails, data can take another path. However, it is expensive and complex to install.

In Simplex Mode, data transmission is unidirectional (one-way only). Example: A keyboard sending data to a computer (computer cannot send back to keyboard).

In Half-Duplex Mode, data can flow in both directions, but not simultaneously. Example: Walkie-Talkie, where only one person can speak at a time.

In Full-Duplex Mode, data can flow in both directions simultaneously. Example: Telephone conversation or mobile phone call.

The OSI (Open Systems Interconnection) Model is a standard framework describing how network protocols interact. It consists of 7 layers: Physical, Data Link, Network, Transport, Session, Presentation, and Application.

Physical Layer (Layer 1): Transmits raw data bits over a medium (cables).
Data Link Layer (Layer 2): Handles error detection and node-to-node transfer (Switches operate here).

Network Layer (Layer 3): Handles logical addressing (IP) and routing (Routers operate here).
Transport Layer (Layer 4): Ensures reliable data delivery and error checking (TCP/UDP).

Session Layer: Manages connections.
Presentation Layer: Translates/encrypts data.
Application Layer (Layer 7): Provides services to user apps (Browsers, Email clients).

IPv4 (Internet Protocol version 4) uses 32-bit addresses represented in decimal notation (e.g., 192.168.1.1). It allows for ~4.3 billion unique addresses, which are now running out.

IPv6 (Internet Protocol version 6) uses 128-bit addresses represented in hexadecimal notation. It provides an almost infinite number of addresses to solve the IPv4 exhaustion problem.

Network Security involves measures to protect data and networks from unauthorized access, misuse, or theft. Key goals include Confidentiality, Integrity, and Availability (CIA).

Malware: Malicious software like viruses and worms.
Phishing: Deceptive attempts (e.g., fake emails) to steal sensitive info.
DoS (Denial of Service): Overwhelming a network to make it unavailable.

Authentication verifies the identity of a user, typically using passwords, biometrics (fingerprint), or multi-factor authentication (MFA).

Encryption: Encoding data so only authorized parties can read it.
Firewall: A security system that monitors and controls incoming/outgoing network traffic based on security rules.