Computer Network & Network Security System — Introduction to Computer Networks and Physical Layer, NEC licence examination syllabus (Nepal Engineering Council).
Before you learn any protocol, ask this one question first: why can't two computers just... talk?
Here's a scenario worth actually sitting with for a second. You send a WhatsApp message from Kathmandu to a friend in Pokhara. That message travels through your phone's Wi-Fi, into a router, across fibre-optic cables possibly running under mountains, through servers owned by companies you've never heard of, and lands on your friend's phone — in under a second. Nobody designed a single machine to do all of that. Instead, thousands of different devices, made by different companies, running different software, all agreed on a common set of rules for how to pass data along. That agreement is what a networking model is.
Think of it like international mail. A letter from Nepal to Canada passes through Nepal Post, an airline, Canada Post, and a local delivery van — none of which were built by the same company, yet the letter arrives. That only works because everyone follows the same "envelope and address" convention. Computer networks work on exactly this idea.Before protocols, there's a simpler question: physically, how are devices even arranged? This is called the topology of the network.
WhatEvery device connects to one single shared cable running down the middle.
WhereOld-school Ethernet setups (mostly historical now).
WhyCheapest possible wiring — but if that one cable breaks anywhere, the ENTIRE network goes down at once.
WhatEvery device connects individually to one central hub/switch.
WhereAlmost every home and office network today.
WhyOne device failing doesn't affect anyone else — but the central hub becomes a single point of failure if IT dies.
WhatEach device connects to exactly two neighbours, forming a closed loop.
WhereOlder Token Ring networks (IEEE 802.5, covered later in this subject).
WhyData has a predictable path around the loop — but one broken link can disrupt the whole ring unless it supports a dual-ring backup.
WhatEvery device connects directly to every other device.
WhereCritical backbone networks (like core internet routers) where downtime isn't acceptable.
WhyExtremely reliable — multiple paths mean one broken link barely matters — but wiring cost explodes as you add devices.
By size, networks are also classified as LAN (Local Area Network — one building, like your college lab), MAN (Metropolitan Area Network — spans a city), and WAN (Wide Area Network — spans countries; the internet itself is the biggest WAN).
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