Pariksha SaathiLesson 9 of 10 · Computer Knowledge
Computer Networks, the OSI Model, Internet Protocols and Email
Drafted with AI assistance and fact-checked on 28 October 2026; not yet read line by line by a human editor. Spotted a mistake? Tell us. Editorial standards & corrections
Networks and their sizes
A computer network is two or more computers connected to share data, printers and an internet connection. A PAN (Personal Area Network) covers a few metres around one person, as when a phone links to earphones over Bluetooth. A LAN (Local Area Network) covers a room, building or campus, is fast and privately owned, and uses Ethernet cables (the IEEE 802.3 standard) or Wi-Fi (IEEE 802.11). A MAN (Metropolitan Area Network) covers a city, like a cable TV network. A WAN (Wide Area Network) spans countries and continents through leased lines and satellites; the internet is the largest WAN. An intranet is a private network inside an organisation built with internet technology, and an extranet opens part of it to outside partners.
Topologies and media
Topology is the layout of a network. In a bus all computers share one backbone cable, so a break in it stops the network. In a star every computer connects to a central hub or switch; it is the most common LAN layout and easy to extend, but everything stops if the centre fails. In a ring each computer connects to two neighbours and data travels round in one direction. In a mesh every device connects to every other, which is the most reliable and the costliest; a full mesh of n devices needs n(n-1)/2 links. Data travels over twisted-pair cable (RJ-45 connectors for Ethernet), coaxial cable (cable TV), optical fibre (light pulses, the fastest and least affected by interference) or wireless links. Transmission is simplex (one way, like radio broadcast), half-duplex (both ways in turn, like a walkie-talkie) or full-duplex (both ways at once, like a telephone call).
Network devices
A NIC (network interface card) connects a computer to a network and carries its MAC address. A repeater regenerates a weak signal. A hub sends incoming data out of all its ports, while a switch sends it only to the port of the destination computer, using MAC addresses. A bridge joins two LAN segments. A router connects different networks and forwards packets along the best path using IP addresses. A modem (modulator-demodulator) converts digital signals to analog for telephone lines and back again. A gateway connects networks that use different protocols, acting as a protocol converter.
The OSI model
The OSI (Open Systems Interconnection) reference model, published by the International Organization for Standardization (ISO) in 1984, divides network communication into seven layers. From the bottom they are 1 Physical (bits, cables, hubs, repeaters), 2 Data Link (frames, MAC addresses, switches, bridges), 3 Network (packets, IP addresses, routing, routers), 4 Transport (end-to-end delivery and error control, TCP and UDP), 5 Session (opening, managing and closing sessions), 6 Presentation (translation, encryption, compression) and 7 Application (user services such as HTTP, FTP, SMTP and DNS). The internet itself runs on the simpler four-layer TCP/IP model.
Protocols and addresses
A protocol is a set of rules for communication. TCP (Transmission Control Protocol) delivers data reliably and in order, UDP sends it faster without that guarantee, and IP (Internet Protocol) addresses and routes packets; TCP/IP was designed by Vint Cerf and Bob Kahn, and ARPANET switched to it on 1 January 1983. HTTP (HyperText Transfer Protocol, port 80) carries web pages, and HTTPS (port 443) carries them encrypted with SSL/TLS. FTP (port 21) transfers files. SMTP (port 25) sends email, POP3 (port 110) downloads mail to one device, usually removing it from the server, and IMAP (port 143) keeps mail on the server and keeps every device in sync. An IPv4 address is 32 bits written as four numbers from 0 to 255, such as 192.168.1.1, and an IPv6 address is 128 bits written in hexadecimal. A MAC address is a 48-bit physical address fixed in the network card. DNS (Domain Name System, port 53) translates domain names into IP addresses.
The web, browsers and search
The World Wide Web, proposed by Tim Berners-Lee at CERN in 1989, is a system of linked pages written in HTML. A URL (Uniform Resource Locator) such as https://www.example.com/index.html has a protocol, a domain name and a path; domains end in suffixes such as .com, .org, .edu, .gov and the country code .in. A web browser such as Chrome, Firefox, Edge or Safari displays pages; Mosaic (1993) was the first popular graphical browser. A search engine indexes pages so they can be found; Archie (1990), which indexed FTP files, is regarded as the first internet search engine, and Google was founded in 1998.
Email and internet history
Ray Tomlinson sent the first network email in 1971 and chose the @ sign, so addresses read user@domain. The To field holds the main recipients, CC (carbon copy) sends copies that every recipient can see, and BCC (blind carbon copy) sends copies whose recipients are hidden from the others. The internet grew from ARPANET, built by the Advanced Research Projects Agency of the US Defense Department; its first message went from UCLA to the Stanford Research Institute on 29 October 1969. In India, ERNET linked education and research institutions from 1986, and VSNL opened public internet service on 15 August 1995.
Key facts
- PAN (Bluetooth, a few metres), LAN (building, Ethernet, Wi-Fi), MAN (city), WAN (countries); the internet is the largest WAN.
- Star topology uses a central hub or switch; bus uses one backbone cable; in a ring data goes one way; mesh is the most reliable and needs n(n-1)/2 links.
- A hub broadcasts to all ports; a switch sends to one port using MAC addresses; a router links different networks using IP addresses; a gateway joins networks with different protocols.
- OSI layers from bottom to top: Physical, Data Link, Network, Transport, Session, Presentation, Application (mnemonic: Please Do Not Throw Sausage Pizza Away).
- The router works at the Network layer, the switch and bridge at the Data Link layer, and the hub and repeater at the Physical layer.
- Ports: HTTP 80, HTTPS 443, FTP 21, SMTP 25, POP3 110, IMAP 143, DNS 53.
- SMTP sends mail; POP3 downloads it to one device; IMAP keeps it on the server and syncs it across devices.
- IPv4: 32 bits, four numbers from 0 to 255 (192.168.1.1); IPv6: 128 bits in hexadecimal; MAC address: 48 bits, fixed in the network card.
- CC recipients are visible to everyone; BCC recipients are hidden from all the other recipients.
- ARPANET's first message: 29 October 1969, UCLA to Stanford Research Institute; Ray Tomlinson's email and the @ sign: 1971; public internet in India by VSNL: 15 August 1995.
Quick revision
PAN, LAN (Ethernet 802.3, Wi-Fi 802.11), MAN, WAN; bus, star, ring, mesh (n(n-1)/2 links); hub, switch (MAC), router (IP), modem, gateway; OSI (ISO, 1984) 7 layers: physical, data link, network, transport, session, presentation, application; TCP/IP (Cerf, Kahn); HTTP 80, HTTPS 443, SMTP 25, POP3 110, IMAP 143; IPv4 32-bit, IPv6 128-bit, MAC 48-bit; DNS; ARPANET 1969; email 1971; VSNL 1995; CC visible, BCC hidden.