Offices, banks and government services now run on networks, so network questions appear in Kharidar, Nayab Subba, Computer Operator and Computer Officer papers. This note covers network types, topologies, transmission media, devices, the OSI and TCP/IP models, addresses and common protocols.
1. What is a computer network?
A computer network is a group of computers and devices connected together to share data, resources and services. Benefits are resource sharing (one printer or internet line for many users), communication (email, chat, video calls), data sharing and central backup. A computer that gives services is a server; a computer that uses them is a client. In a peer-to-peer network every computer is equal.
2. Types of network by area
| Type | Full form | Area covered | Example |
|---|---|---|---|
| PAN | Personal Area Network | A few metres around one person | Bluetooth between phone and earphones |
| LAN | Local Area Network | One room, building or campus | Network of an office or school |
| MAN | Metropolitan Area Network | A city | Cable TV or city-wide network |
| WAN | Wide Area Network | Countries and continents | The internet, the largest WAN |
3. Network topology
Topology is the physical or logical layout in which devices are connected.
| Topology | How it is connected | Key point |
|---|---|---|
| Bus | All devices on one main cable (backbone) | Cheap, but a break in the backbone stops the network |
| Star | Every device connected to a central hub or switch | Most common in LANs; if the hub fails, all stop |
| Ring | Each device connected to two others in a circle | Data moves in one direction; one failure can break the ring |
| Mesh | Every device connected to every other device | Most reliable but most costly; n(n-1)/2 links |
| Tree | Star networks joined in a hierarchy | Used in large organisations |
| Hybrid | A mix of two or more topologies | Flexible |
4. Transmission media
- Twisted pair cable: two insulated copper wires twisted together; used in LANs with the RJ-45 connector (telephone lines use RJ-11). UTP means Unshielded Twisted Pair.
- Coaxial cable: a copper core with a metal shield; used for cable TV.
- Optical fibre: carries data as light using total internal reflection; the fastest, with very little loss and no electrical interference.
- Wireless (unguided) media: radio waves, microwaves, satellite, infrared, Bluetooth and Wi-Fi.
5. Network devices
| Device | Work |
|---|---|
| NIC (Network Interface Card) | Connects a computer to a network; has a unique MAC address |
| Hub | Sends incoming data to all connected devices (not intelligent) |
| Switch | Sends data only to the correct device using the MAC address |
| Router | Connects different networks and chooses the best path using IP addresses |
| Modem | Modulator-Demodulator; converts digital signals to analogue and back |
| Repeater | Strengthens (regenerates) a weak signal to cover longer distance |
| Bridge | Joins two LAN segments |
| Gateway | Connects networks that use different protocols |
| Access point | Lets wireless devices join a wired network (Wi-Fi) |
6. OSI and TCP/IP models
The OSI (Open Systems Interconnection) model, made by ISO, divides network communication into 7 layers, from bottom to top:
| No. | OSI layer | Example of work or device |
|---|---|---|
| 1 | Physical | Cables, signals, hub, repeater |
| 2 | Data Link | MAC address, switch, bridge |
| 3 | Network | IP address, routing, router |
| 4 | Transport | TCP and UDP, error control |
| 5 | Session | Opens and closes sessions |
| 6 | Presentation | Encryption, compression, data format |
| 7 | Application | HTTP, FTP, SMTP, DNS |
Memory aid from layer 1 to 7: Please Do Not Throw Sausage Pizza Away. The TCP/IP model, used on the internet, has 4 layers: Network Access (Link), Internet, Transport and Application.
7. Addresses and protocols
- IP address: a logical address of a device on a network. IPv4 is 32-bit, written as four numbers from 0 to 255, for example 192.168.1.1. IPv6 is 128-bit, written in hexadecimal.
- MAC address: the physical address of a network card, 48-bit, written as six pairs of hexadecimal digits.
- DNS (Domain Name System) changes a domain name such as loksewapathsala.com into an IP address.
- DHCP gives IP addresses to devices automatically.
| Protocol | Full form | Use |
|---|---|---|
| HTTP / HTTPS | HyperText Transfer Protocol (Secure) | Opening web pages; HTTPS is encrypted |
| FTP | File Transfer Protocol | Uploading and downloading files |
| SMTP | Simple Mail Transfer Protocol | Sending email |
| POP3 / IMAP | Post Office Protocol 3 / Internet Message Access Protocol | Receiving email |
| TCP | Transmission Control Protocol | Reliable, connection-oriented delivery |
| UDP | User Datagram Protocol | Fast, connectionless delivery, used for video calls and games |
| Telnet / SSH | Teletype Network / Secure Shell | Remote login (SSH is secure) |
8. Transmission modes and speed
| Mode | Direction | Example |
|---|---|---|
| Simplex | One direction only | Radio and TV broadcast, keyboard to computer |
| Half duplex | Both directions, but one at a time | Walkie-talkie |
| Full duplex | Both directions at the same time | Telephone and mobile call |
Network speed (data rate) is measured in bits per second (bps): kbps, Mbps and Gbps. Bandwidth is the maximum amount of data a connection can carry in a given time.
📌 Key points to remember
- LAN covers a building, MAN a city and WAN countries; the internet is the largest WAN.
- Star topology is the most common; mesh is the most reliable and costly.
- Optical fibre carries data as light and is the fastest medium.
- A hub sends data to all, a switch to the right device, and a router connects different networks.
- Modem means modulator-demodulator.
- OSI has 7 layers (Physical to Application); TCP/IP has 4 layers.
- IPv4 is 32-bit, IPv6 is 128-bit and a MAC address is 48-bit.
- SMTP sends email, POP3 or IMAP receive it; DNS turns domain names into IP addresses.
- Simplex: one way; half duplex: walkie-talkie; full duplex: telephone.
Test yourself: Computer Network Notes
20 questions on What is a computer network?, Network devices and Transmission modes and speed.