What is a computer network?
A computer network is two or more devices (computers, phones, printers) connected so they can share data and resources. Each device on a network is called a node or host.
Why network? To share files, printers and internet access, to talk (email, video calls), and to keep data in one place that many people can use.
Evolution: ARPANET, NSFNET and the Internet
- ARPANET (1969): started by ARPA, a research agency of the US defence department. It first linked 4 university computers. It was the first network to use packet switching on a large scale, and later adopted the TCP/IP protocol (1983).
- NSFNET (1986): made by the National Science Foundation to link universities and research centres with a faster backbone (main high-speed line). It was used for education and research.
- Internet (1990s onward): ARPANET, NSFNET and many private networks joined using TCP/IP. The Internet is a network of networks that spans the whole world. The World Wide Web (1991) made it easy to use through web pages.
Components of data communication
Data communication means sending data from one device to another over some medium.
- Sender: the device that sends the data (your phone).
- Receiver: the device that gets it (your friend's phone).
- Message: the data itself: text, picture, audio, video.
- Communication media: the path the message travels on: a cable or wireless signals.
- Protocol: the set of rules both sides agree to follow, like the format of data and how errors are handled. Without a common protocol they cannot understand each other.
Bandwidth and data rate
Bandwidth is the range of frequencies a channel can carry: highest frequency minus lowest. It is measured in hertz (Hz, kHz, MHz, GHz). More bandwidth means the channel can carry more data.
Data rate (or bit rate) is the number of bits sent per second. Units: bps, Kbps, Mbps, Gbps. 1 Kbps = 1000 bps; 1 Mbps = 1000 Kbps; 1 Gbps = 1000 Mbps.
Time to send = data size (in bits) ÷ data rate. Remember 1 byte = 8 bits. A 10 MB file on a 40 Mbps line: 10 × 8 = 80 Mb ÷ 40 Mbps = 2 seconds (ignoring delays).
IP address
An IP address (Internet Protocol address) is a unique number given to each device on a network, so data knows where to go, like a house address for letters.
- IPv4: 32 bits, written as four numbers from 0 to 255 separated by dots, e.g.
192.168.1.10. About 4.3 billion addresses. - IPv6: 128 bits, written as eight groups of hexadecimal digits separated by colons, e.g.
2001:db8::1. Made because IPv4 addresses were running out.
Every network card also has a fixed MAC address (48 bits) set by the maker; the IP address can change when you join a different network.
Switching: circuit switching and packet switching
Switching is how data finds its way through the network from sender to receiver.
Circuit switching
A fixed path (circuit) is set up between sender and receiver before data flows, and it stays booked for the whole talk. Example: old landline telephone calls. Good: steady, in-order data. Bad: the line is wasted during silence; setup takes time.
Packet switching
Data is cut into small packets. Each packet carries the sender and receiver address and a sequence number. Packets travel on their own, maybe by different routes, and are joined in order at the receiver. Example: the Internet. Good: lines are shared well, a broken link can be avoided. Bad: packets may arrive out of order or be delayed.
| Circuit | Packet | |
|---|---|---|
| Path | one fixed path, booked | any free path per packet |
| Setup | needed first | not needed |
| Use of lines | wasteful | efficient |
| Order | always in order | re-ordered at the end |
Try it: find your own IP address and speed
On a phone, open Wi-Fi settings and tap your network: you will see an IP address like 192.168.x.x. On a computer, type ipconfig (Windows) or ip addr (Linux) in the terminal. Then run a speed test in a browser and note the download speed in Mbps. Work out how many seconds a 25 MB video should take to download. Check the answer with the slider in the 3D free-play step.
Key formulas and definitions
- Five parts: sender, receiver, message, media, protocol
- Time = size in bits ÷ data rate; 1 byte = 8 bits
- 1 Kbps = 10³ bps, 1 Mbps = 10⁶ bps, 1 Gbps = 10⁹ bps
- IPv4 = 32 bits (4 numbers 0–255) · IPv6 = 128 bits
Worked examples
1. Arrange in time order: Internet, ARPANET, NSFNET.
ARPANET (1969) → NSFNET (1986) → Internet (grew in the 1990s by joining networks).
2. In an email from Asha to Ravi through Wi-Fi, name the five components.
Sender: Asha's device. Receiver: Ravi's device. Message: the email. Media: Wi-Fi radio waves (and cables beyond). Protocol: SMTP/TCP/IP rules.
3. How long does a 5 MB file take on a 10 Mbps line?
5 MB = 5 × 8 = 40 megabits. Time = 40 ÷ 10 = 4 seconds.
4. A channel carries frequencies from 300 Hz to 3400 Hz. Find its bandwidth.
Bandwidth = 3400 − 300 = 3100 Hz.
5. Is 192.168.1.256 a valid IPv4 address?
No. Each of the four numbers must be from 0 to 255, and 256 is too big.
6. Which switching suits a live video call on the Internet, and which suits an old landline call? Why?
Internet video uses packet switching: packets share lines with others and find free routes. An old landline uses circuit switching: one line is booked for the whole call, so the voice arrives in order without sharing.
Common mistakes
- Mixing bandwidth (Hz, range of frequencies) with data rate (bits per second).
- Forgetting to multiply bytes by 8 when finding time from Mbps.
- Saying ARPANET and the Internet are the same; ARPANET was one early network that the Internet grew from.
- Thinking packets always take the same path; each packet can take a different route.