Revision Summary: Computer Networks (NCERT Class 12)
1. Chapter at a Glance
- A computer network is an interconnection among two or more computers or computing devices that allows sharing of data and resources.
- Nodes in a network are devices that can receive, create, store or send data; networking devices (modem, hub, switch, router, gateway, etc.) connect multiple computers.
- Networks are classified by geographical area and data transfer rate as PAN (≈10 m), LAN (up to 1 km, 10–1000 Mbps), MAN (city/town, 30–40 km) and WAN (countries/continents; Internet is the largest WAN).
- Data travel as packets; signals are carried over wired (cables, fibre) or wireless media and may need regeneration by repeaters.
- Network topologies describe the physical arrangement of nodes: Mesh, Ring, Bus, Star and Tree (Hybrid).
- Every node is identified by a permanent MAC address (48-bit hexadecimal on NIC) and a changeable IP address (IPv4: 32-bit dotted decimal; IPv6: 128-bit hexadecimal).
- The World Wide Web is an ocean of interlinked web pages accessed over the Internet using HTML, URI/URL and HTTP/HTTPS; domain names are resolved to IP addresses by DNS servers.
2. Key Terms and Definitions
- Network: A group of two or more similar things or people interconnected with each other.
- Computer network: An interconnection among two or more computers or computing devices that allows sharing of data and resources.
- Node: Any device that is part of a network and can receive, create, store or send data to different network routes (e.g., computer, printer, modem, router).
- Packet: Smaller chunks into which data is divided for transmission over a network.
- PAN (Personal Area Network): Network formed by connecting personal devices within ≈10 metres; may be wired or wireless (WPAN via Bluetooth).
- LAN (Local Area Network): Network covering a limited area (room, floor, campus) up to 1 km with high data rates (10–1000 Mbps) using Ethernet cables or Wi-Fi.
- MAN (Metropolitan Area Network): Extended LAN covering a city or town (30–40 km); examples include cable TV networks.
- WAN (Wide Area Network): Network connecting LANs/MANs across countries or continents; the Internet is the largest WAN.
- Modem (MOdulator DEModulator): Device that converts digital data to analog signals (modulation) at the sender and analog signals back to digital data (demodulation) at the receiver.
- Ethernet card / NIC: Network Interface Card mounted on the motherboard that acts as an interface between the computer and a wired network; each has a unique MAC address.
- RJ-45: Eight-pin connector used exclusively with Ethernet cables.
- Repeater: Analog device that regenerates weakened signals on a cable (typically beyond 100 m).
- Hub: Device that connects multiple devices and broadcasts incoming data to all other ports; collisions occur if data arrives simultaneously.
- Switch: Intelligent device that forwards data only to the intended recipient by looking up the destination address; drops corrupted packets.
- Router: Device that analyses data, decides routing paths and can connect networks of different types; can repackage packets.
- Gateway: Entry/exit point of a network; all incoming/outgoing data passes through it; usually integrates routing and firewall functions.
- Topology: The arrangement of computers and other peripherals in a network.
- Mesh topology: Every device is connected to every other device.
- Ring topology: Each node is connected to exactly two other nodes forming a unidirectional ring.
- Bus topology: All devices share a single backbone cable (bus); data travels in both directions.
- Star topology: Every device is connected to a central networking device (hub or switch).
- Tree/Hybrid topology: Hierarchical combination of two or more basic topologies (e.g., star connected via bus).
- MAC address: 48-bit permanent hexadecimal hardware address engraved on the NIC (first 24 bits = manufacturer OUI).
- IP address: Unique logical address assigned to each node using Internet Protocol (IPv4: 32-bit dotted decimal; IPv6: 128-bit hexadecimal groups separated by colons).
- DNS (Domain Name System): Hierarchical system that resolves domain names to corresponding IP addresses using DNS servers.
- Domain name resolution: Process of converting a domain name into its IP address via a DNS server.
- WWW (World Wide Web): Collection of interlinked web pages and resources stored on servers and accessed over the Internet using HTML, URI/URL and HTTP/HTTPS.
3. Syntax and Constructs
No programming statements, functions or code constructs are taught in this chapter.
4. Algorithms and Worked Logic
Domain name resolution (procedure a student must be able to describe)
Key steps (as per NCERT):
1. User enters a URL/domain name in the browser.
2. HTTP protocol sends a request to a DNS server.
3. DNS server looks up the corresponding IP address (or refers to another DNS server in the hierarchy).
4. IP address is returned to the browser.
5. HTTP uses the IP address to retrieve the web page/resource and loads it in the browser.
5. Common Errors and Exam Pitfalls
- Confusing PAN/LAN/MAN/WAN ranges or examples (e.g., stating Internet is a LAN).
- Mixing up devices: hub broadcasts to all while switch forwards selectively; repeater only regenerates signals.
- Stating MAC address is changeable or IP address is permanent.
- Writing incorrect bit lengths (MAC = 48 bits, IPv4 = 32 bits, IPv6 = 128 bits) or address formats.
- Claiming “Web” and “Internet” are the same; forgetting that WWW uses HTML + URI + HTTP while Internet is the underlying network.
- Forgetting that tree topology is a hierarchical combination of basic topologies.
- In diagrams, drawing incorrect connections (e.g., showing a bus in star topology) or omitting central device in star topology.
- Missing that gateway acts as entry/exit point and often integrates firewall functions.