Chapter 3 — Computer Network

3.1Basic Networking: definition, types, topologies

Definition

A computer network is a collection of two or more computers and devices connected together to share resources (files, printers, Internet) and communicate.

Think of a building with shared WhatsApp group. One person cooks extra daal-bhat (a file) and announces it on the group. Everyone who wants some can come pick it up. That's a network — shared resources, shared conversation.

Types of networks (by size)

PAN (Personal Area Network) — 1–10 m. Your phone + Bluetooth headphones.
LAN (Local Area Network) — one building. Your office / cyber café / school.
MAN (Metropolitan Area Network) — one city. ISP network across Kathmandu.
WAN (Wide Area Network) — country or global. The Internet itself.

Topologies — the shape of a network

Topology	Shape	Vibe
Bus	All devices on one single cable	Cheap, but one cable break = whole network down
Star	All devices connect to a central hub/switch	Most common in offices; if hub fails, network dies
Ring	Devices in a closed loop, data goes round	One break = whole ring down
Mesh	Every device connects to every other	Super reliable, expensive. Used in military, Internet backbone
Tree	Hierarchy — like branches from a trunk	Combines bus + star; used in large networks
Hybrid	Mix of two or more above	Real-world networks are usually this

Star topology = electricity board in your house. Every switch, fan, light connects back to the main board. If one fan dies, the rest keep running. But if the main board trips, everything's off.

3.2Connectivity and media: Network cables and connectors

Transmission media — wired (guided)

Twisted pair cable — pairs of copper wires twisted together.
- UTP (Unshielded) — common LAN cable. Cat5e, Cat6.
- STP (Shielded) — has extra shielding against interference.
Coaxial cable — single copper core with shielding. Used for cable TV, older networks.
Fibre optic cable — transmits data as light pulses through glass. Very fast, long distance, expensive. Used for internet backbone.

Transmission media — wireless (unguided)

Radio waves — Wi-Fi, cellular (4G/5G).
Microwaves — satellite links, point-to-point towers.
Infrared — TV remotes, short-range.
Bluetooth — short-range personal devices.

Connectors

RJ-45 — the familiar Ethernet plug, used with twisted pair for LAN.
RJ-11 — smaller, used for telephone and old DSL.
BNC — for coaxial cable.
ST / SC / LC — for fibre optic cables.

Copper cable = sending letters through a postman. Fibre optic = sending the same message as a laser flash through glass. Light always wins the race.

3.3Network Devices (Hub, Switch, Router, Gateway)

Device	What it does	Intelligence
Hub	Sends incoming signal to every port. Dumb repeater.	None
Switch	Sends data only to the port where the target device lives. Uses MAC addresses.	Smart within a LAN
Router	Connects different networks, picks the best path between them. Uses IP addresses.	Smart across networks
Gateway	Connects networks that use different protocols (translator between two systems).	Very smart — translates
Modem	Converts digital ↔ analog so your ISP's line can reach your computer.	Simple converter
Bridge	Connects two LAN segments into one.	Moderate
Repeater	Boosts a weak signal so it can travel further.	None (amplifier)
NIC	Network Interface Card — the actual Ethernet/Wi-Fi chip in your computer.	N/A (component)

Router vs Switch = postman vs office peon. The office peon (switch) only delivers mail inside one office — if it's for someone in this building, they know which desk. The postman (router) picks up mail that needs to go to another city and decides which road is fastest.

Hub = that uncle at a wedding who shouts every announcement to the whole room even if it's only for the bride. Switch = the MC who walks over to the right table and whispers it.

3.4Network cabling and cable testing

Crimping a cable

To make your own Ethernet cable: strip the outer jacket, untwist the 8 wires, arrange them in the T568A or T568B color order, trim, insert into an RJ-45 connector, and use a crimping tool to press the connector shut.

Two main wiring standards (colors)

T568B (most common) — Orange/white, Orange, Green/white, Blue, Blue/white, Green, Brown/white, Brown.
T568A — Green/white, Green, Orange/white, Blue, Blue/white, Orange, Brown/white, Brown.

Straight-through vs Crossover

Straight-through — same standard on both ends. Connects different devices: PC ↔ switch, switch ↔ router.
Crossover — T568A on one end, T568B on the other. Connects same-type devices: PC ↔ PC, switch ↔ switch (in older gear; modern gear auto-detects).

Cable testing

A cable tester has two small boxes: plug each end of the cable into one. LEDs light up in sequence to confirm every wire is connected correctly. Broken wire = missing LED.

Cable tester = doctor's stethoscope for your network cable. Instead of heartbeats it listens for electric signals on all 8 wires.

3.5IP Addressing

An IP address (Internet Protocol address) is a unique number assigned to every device on a network. It's how the network knows where to send packets.

IP address = your home's ward + tole + house number. Without it, the postman has no idea where to deliver your Daraz parcel.

IPv4

32-bit address, written as four numbers (0–255) separated by dots: 192.168.1.10.
About 4.3 billion addresses total — we've nearly run out.

IPv6

128-bit address, written as 8 groups of hex digits: 2001:0db8:85a3::7334.
Huge address space — basically unlimited.

Classes of IPv4

Class	Range (first octet)	Used for
A	1–126	Very large networks
B	128–191	Medium networks
C	192–223	Small networks (most common)
D	224–239	Multicast
E	240–255	Research/experimental

Public vs private IP

Public IP — unique on the entire Internet. Assigned by your ISP.
Private IP — reused inside local networks. Ranges: 10.x.x.x, 172.16.x.x – 172.31.x.x, 192.168.x.x.

Static vs dynamic

Static — manually assigned, doesn't change. For servers.
Dynamic — given out automatically by DHCP. For home devices.

Special addresses

127.0.0.1 — loopback; always refers to "this computer."
0.0.0.0 — "any address" / unspecified.
Subnet mask — tells you which part of the IP is the network and which is the host, e.g. 255.255.255.0.

3.6E-mail, Internet, Extranet, Intranet, WWW

Internet

A global network of networks. Billions of devices connected through ISPs, routers, and undersea cables.

Intranet

A private network inside an organization, using Internet-style tools (web pages, email) but not accessible from outside. Example: an office's internal portal.

Extranet

An intranet that selectively lets in outsiders — typically suppliers, vendors, partners. Example: a bank's portal for its partner merchants.

Intranet = your family's private WhatsApp group. Extranet = same group but with your in-laws added. Internet = a group with the whole country in it.

E-mail (Electronic mail)

A method of exchanging digital messages over a network. Requires an email address (you@gmail.com) and a server.

To, Cc, Bcc, Subject, Body, Attachment — standard fields.
Protocols: SMTP (sending), POP3 / IMAP (receiving).

World Wide Web (WWW)

The WWW is a collection of interlinked web pages (HTML documents) accessed via the Internet using HTTP/HTTPS. It's not the same as the Internet — the Internet is the infrastructure, the WWW is one service that runs on it.

Invented by Tim Berners-Lee in 1989 at CERN.
Needs: a web browser (client) + a web server + a URL (https://example.com).

Internet = the road system. WWW = the shops on those roads. Email, file transfer, online games — those are other shops on the same roads, not part of the WWW.

3.7Internet browsers

A web browser is software that fetches web pages from servers and displays them. You type a URL → browser sends an HTTP request → server sends HTML/CSS/JS back → browser renders it.

Popular browsers

Google Chrome — fast, dominates market share.
Mozilla Firefox — open-source, privacy-focused.
Microsoft Edge — replaced Internet Explorer, built on Chromium.
Internet Explorer — Microsoft's old browser, now discontinued.
Safari — Apple's browser.
Opera — known for Opera Mini on slow connections.

Key parts of a browser

Address bar — where you type the URL.
Tabs — many pages at once.
Bookmarks — saved links.
History — list of visited pages.
Cache — saved copy of pages for speed.
Cookies — small data files that remember you (login sessions, preferences).

URL anatomy

https://www.example.com.np/page?id=5

https — protocol (secure HTTP).
www.example.com.np — domain name.
/page — path to a specific resource.
?id=5 — query parameter.

URL = a courier tracking number + full address. The protocol is "which courier company" (DHL vs Aramex), the domain is the street, the path is the flat number.

🌐 Computer Network