Computer Networks

Computer Network: • A computer network is a collection of interconnected computers and devices that can communicate with each other and share resources (data, hardware, software, internet connectivity). • Networks enable resource sharing, communication, data storage, and collaborative work. Types of Networks Based on Geographical Area: 1. PAN (Personal Area Network): • Coverage: A few metres — personal space of an individual. • Connects personal devices (smartphone, laptop, tablet, smartwatch, wireless keyboard). • Technology: Bluetooth, Infrared, USB, Zigbee. • Example: A smartphone connected to a Bluetooth headset. 2. LAN (Local Area Network): • Coverage: A building or campus — up to a few kilometres. • Connects computers in a school, office, or home. • Technology: Ethernet (wired), Wi-Fi (wireless — WLAN). • High speed, low cost, private ownership. • Example: School computer lab network; home Wi-Fi network. 3. MAN (Metropolitan Area Network): • Coverage: A city or metropolitan area — up to ~50 km. • Connects multiple LANs across a city. • Often used by municipalities, cable TV operators, or large organisations with multiple offices in a city. • Example: City-wide broadband network; cable TV network. 4. WAN (Wide Area Network): • Coverage: Countries, continents, or globally. • Connects computers over long distances — often using leased telecommunication lines, satellite links, or undersea cables. • The Internet is the world's largest WAN. • Lower speed than LAN but much greater coverage; typically operated by telecom companies. Basic Components of a Network: 1. Nodes: Computers, servers, printers, smartphones — any device connected to the network. 2. Network Interface Card (NIC): Hardware in each device that connects it to the network — has a unique MAC address. 3. Transmission Medium: The path data travels — twisted pair cable, coaxial cable, optical fibre, or wireless (radio waves). 4. Hub / Switch: Connects multiple devices in a network. A switch is smarter — it sends data only to the intended device. 5. Router: Connects networks together (e.g., LAN to the Internet) and routes data between them. 6. Modem: Modulates/demodulates signals for transmission over telephone lines. 7. Server: A powerful computer that provides services (files, web pages, email) to other nodes (clients).

Network Topology: • Network topology describes the physical or logical arrangement of computers and connections in a network. 1. Bus Topology: • All devices are connected to a single central cable called the 'bus' or 'backbone'. • Data travels in both directions along the bus. • A terminator at each end absorbs signals to prevent bouncing. Advantages: • Simple and inexpensive to set up. • Less cabling required. • Easy to add new devices. Disadvantages: • A break in the bus cable disables the entire network. • Performance degrades as more devices are added (collisions increase). • Difficult to troubleshoot — locating a fault on the bus is hard. • Limited cable length and number of devices. 2. Star Topology: • All devices are connected to a central hub or switch via individual cables. • All communication passes through the central device. Advantages: • Easy to install and manage. • A fault in one cable or device does not affect others — easy to isolate failures. • Easy to add or remove devices without disrupting the network. • Good performance (switch sends data only to the intended device). Disadvantages: • Requires more cabling than bus topology. • Central hub/switch is a single point of failure — if it fails, the entire network goes down. • More expensive due to additional hardware (hub/switch). 3. Ring Topology: • Devices are connected in a closed loop — each device connects to exactly two neighbours. • Data travels in one direction (or both in dual-ring) around the ring. • Token Ring uses a 'token' that devices must capture before transmitting. Advantages: • No data collisions — only the device with the token transmits. • Good performance under heavy load. • Equal access for all devices. Disadvantages: • A single break in the ring disables the entire network. • Difficult to troubleshoot. • Adding or removing a device disrupts the network. • Slower than star topology for light loads (token must circulate even when idle).

TCP/IP Protocol Suite: • TCP/IP (Transmission Control Protocol / Internet Protocol) is the foundational set of communication protocols for the Internet and most computer networks. • It defines how data is packaged, transmitted, routed, and received across networks. • TCP/IP is a layered model — protocols at each layer handle specific aspects of communication. The Four Layers of TCP/IP: 1. Application Layer: Protocols for specific applications (HTTP, FTP, SMTP, DNS, POP3). 2. Transport Layer: Reliable (TCP) or fast (UDP) data delivery between applications. 3. Internet Layer: IP addressing and routing — IP protocol routes packets across networks. 4. Network Access (Link) Layer: Physical transmission — Ethernet, Wi-Fi. Key Protocols: 1. HTTP (HyperText Transfer Protocol): • Application layer protocol for transferring web pages (HTML, images, CSS, JavaScript) between a web server and a web browser. • HTTPS is the secure version — uses SSL/TLS encryption. • Default port: 80 (HTTP), 443 (HTTPS). • Example: When you type www.google.com, your browser sends an HTTP GET request to Google's server; the server responds with the webpage content. 2. FTP (File Transfer Protocol): • Application layer protocol for transferring files between computers over a network. • Supports both uploading (PUT) and downloading (GET) of files. • Uses two connections: one for commands (port 21), one for data (port 20). • SFTP (Secure FTP) encrypts transfers. • Example: A web developer uses FTP to upload website files to a web server. 3. DNS (Domain Name System): • Translates human-readable domain names (www.ncert.nic.in) into IP addresses (202.141.128.114) that computers use. • DNS is like the Internet's phone directory. • Without DNS, users would have to type numeric IP addresses. • DNS servers are arranged in a hierarchy: root servers → TLD servers (.com, .in) → authoritative servers. 4. SMTP (Simple Mail Transfer Protocol): • Protocol for sending email from a client to a mail server and between mail servers. • Uses port 25 (or 587 for secure submission). • Works with POP3 (Post Office Protocol) or IMAP for receiving email. • Flow: Sender's email client → SMTP server → Internet → Recipient's SMTP server → POP3/IMAP → Recipient's email client. 5. IP (Internet Protocol): • Assigns unique addresses (IP addresses) to devices and routes packets across networks. • IPv4: 32-bit address (e.g., 192.168.1.1); IPv6: 128-bit (e.g., 2001:db8::1) — needed due to IPv4 exhaustion.