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Beginner · How networks work · topic 1 of 57 · 25 min

What a network is

Explanation

A network is two or more devices (nodes) joined by links so they can share data. Small networks in one building are LANs, networks that join cities are WANs, and the internet is a huge network of networks. Speed is measured in bits per second, so remember that one byte is eight bits when you work out download times.

Why networks exist

A computer on its own can only use what is inside it. Join it to other computers and it can print on a shared printer, open files on a server, send a UPI payment or watch a video stored thousands of kilometres away. A network is simply two or more devices connected so they can exchange data.

Every device on a network is called a node. A node can be a laptop, a phone, a printer, a server, a switch or a router. The connection between two nodes is called a link. A link can be a copper cable, a fibre optic cable or a radio signal such as Wi-Fi.

Think of it like this: Think of the Indian Railways network. Stations are the nodes and tracks are the links. A small local line inside one city is like a LAN. The long-distance lines that join Mumbai to Delhi are like a WAN. Together, all the lines form one national network, just as all the small networks of the world together form the internet.

LAN, WAN and MAN

TypeMeaningExample
LAN (Local Area Network)Devices in one home, office or campus, usually owned by one organisationThe Wi-Fi and wired network of a college computer lab
MAN (Metropolitan Area Network)Joins several sites across one cityA bank linking its branches inside Pune over leased fibre
WAN (Wide Area Network)Joins sites across cities or countries, usually over links rented from a service providerA retail chain joining its Chennai head office to stores in 40 cities

The internet is not one network owned by one company. It is a network of networks: thousands of separate networks run by internet service providers (ISPs) such as Jio, Airtel and BSNL, by cloud companies and by universities, all agreeing to pass traffic to each other using the same rules.

Client-server and peer-to-peer

In the client-server model, one machine (the server) offers a service and many machines (the clients) ask for it. When you open the IRCTC app, your phone is the client and IRCTC servers answer. In the peer-to-peer model, every machine can be both client and server. File sharing with BitTorrent and some video call systems work like this. Most business networks are client-server because it is easier to control and back up one central server.

Bandwidth, throughput and latency

  • Bandwidth: the maximum amount of data a link can carry per second. A 100 Mbps plan has a bandwidth of 100 megabits per second.
  • Throughput: the amount of data that actually gets through per second. It is usually lower than bandwidth because of other users, Wi-Fi interference and protocol overhead.
  • Latency: the time one piece of data takes to travel from sender to receiver, measured in milliseconds (ms). The round trip time (RTT) is the time to go there and come back, which is what ping shows.
Think of it like this: A highway helps here. Bandwidth is the number of lanes. Throughput is how many cars actually pass per minute, which drops in a traffic jam. Latency is how long one car takes to drive from one end to the other. A wide road can still have a long travel time if it is very long.

Bits and bytes

A bit is a single 0 or 1. A byte is 8 bits. Network speeds are given in bits per second (bps, Kbps, Mbps, Gbps), but file sizes are given in bytes (KB, MB, GB). This is the most common source of confusion. A 100 Mbps connection moves at most 100 divided by 8, which is 12.5 megabytes per second.

python
size_mb = 700          # a 700 MB file
speed_mbps = 50        # a 50 Mbps connection
seconds = size_mb * 8 / speed_mbps
print(seconds)         # 112.0 seconds, a little under 2 minutes
Multiply megabytes by 8 to get megabits, then divide by the speed in megabits per second.
Careful: Common mistake: dividing the file size in megabytes straight by the speed in Mbps. That gives an answer 8 times too small. Another mistake is mixing small b (bits) and capital B (bytes): 1 MBps is 8 Mbps. Always convert both to bits first.

Topologies: physical and logical

Topology means the shape of a network. The physical topology is how the cables and devices are really laid out. The logical topology is how data actually flows. They can differ: a Wi-Fi network looks like nothing physically, but logically every device talks through the access point.

TopologyHow it looksWhere you see it
StarEvery device connects to one central device, usually a switchAlmost every office LAN today
Full meshEvery device connects to every other deviceCore routers of a large network, where losing a link must not stop traffic
Partial meshSome devices have several links, others have oneWAN links between important branches
HybridA mix, for example stars on each floor joined by a mesh in the coreA college campus or hospital network

Older topologies such as bus (one shared cable) and ring are mainly of historic interest. A full mesh of n devices needs n times (n minus 1) divided by 2 links, so 5 routers need 10 links and 10 routers need 45. That is why full mesh is used only where the cost is worth it.

Tip: When someone says "the internet is slow", ask which of the three they mean: is the bandwidth too small for the job, is the throughput low because of congestion or bad Wi-Fi, or is the latency high because the server is far away? Each has a different fix.

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What a network is · 1/7

What is a network?

Welcome to your first networking lesson. A network is simply two or more devices joined so they can share data.

Prefer a video by a teacher? Find free videos on “what is a computer network LAN WAN bandwidth latency for beginners” on YouTube ↗

In real life: Planning a backup at a small CA firm in Jaipur

A chartered accountant office wants to back up 20 GB of client files to the cloud every night over a 40 Mbps upload link. The IT person works it out: 20 GB is about 20,000 MB, times 8 is 160,000 megabits, divided by 40 gives 4,000 seconds, a little over an hour. Throughput is usually lower than the plan speed, so she schedules the backup at 11 pm, when nobody is working, and allows two hours.

python
size_mb = 20_000
upload_mbps = 40
print(size_mb * 8 / upload_mbps / 60, "minutes")   # about 66.7 minutes
Bits, not bytes: multiply by 8 before dividing by the speed.

Practice (0/3 done)

Task 1 · Code · Python

Write transfer_seconds(size_mb, speed_mbps) that returns how many seconds it takes to move a file of size_mb megabytes over a link of speed_mbps megabits per second. Remember: 1 byte = 8 bits. Example: transfer_seconds(100, 100) returns 8.0.

Task 2 · Code · Python

A full mesh joins every device to every other device. Write mesh_links(n) that returns how many links a full mesh of n devices needs. Example: mesh_links(5) returns 10.

Task 3 · Practice

Draw the network in your home, hostel or office on paper. Mark every node and every link, and say whether each link is wired or wireless.

Quick quiz

Question 1

1.A bank joins its branches in Mumbai, Delhi and Kolkata using links rented from a provider. What kind of network is this?

Question 2

2.Your plan is 80 Mbps. What is the fastest possible download speed in megabytes per second?

Question 3

3.Which term means the time one packet takes to travel from sender to receiver?

Question 4

4.Every PC on each floor of an office plugs into one switch, and the floor switches connect to two core switches that are linked to each other. Which topology is this best described as?

Question 5

5.Which statement about the internet is correct?

Read more (free)

Open a question to see the answer and why. The first ones are this topic's own quiz, so try the quiz above before you peek.

A bank joins its branches in Mumbai, Delhi and Kolkata using links rented from a provider. What kind of network is this?

Answer: WAN

Joining sites in different cities over provider links is a wide area network.

Your plan is 80 Mbps. What is the fastest possible download speed in megabytes per second?

Answer: 10 MB/s

80 megabits divided by 8 bits per byte is 10 megabytes per second.

Which term means the time one packet takes to travel from sender to receiver?

Answer: Latency

Latency is delay, measured in milliseconds. Bandwidth and throughput measure amount per second.

Every PC on each floor of an office plugs into one switch, and the floor switches connect to two core switches that are linked to each other. Which topology is this best described as?

Answer: Hybrid

Stars on each floor joined by a meshed core is a mix of topologies, called hybrid.

Which statement about the internet is correct?

Answer: It is a network of many separate networks that agree to exchange traffic

Thousands of ISPs and organisations run their own networks and pass traffic to each other.

What does a wireless LAN controller do?Related

Answer: Centrally manages many lightweight access points

A WLC pushes configuration to APs and handles roaming.

From Network devices

Which statement about controller-based networking is true?Related

Answer: Devices still forward traffic; control and management are centralised

The data plane stays on the switches and routers. The controller centralises decisions, configuration and policy.

From Why automate: SDN and controller-based networking

A private app server must download updates from the internet but must not accept connections from it. What does its subnet route to?Related

Answer: A NAT gateway

A NAT gateway lets private servers start outbound connections while blocking inbound ones.

From Virtualisation, VRFs and cloud networking

A PC shows no link light on its network port. Which layer should you check first?Related

Answer: Layer 1

No link light points to the physical layer: cable, port or NIC.

From The OSI model

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Next steps

  1. 4 more topics in “How networks work” before its exam: Network devices, The OSI model, The TCP/IP model and encapsulation, TCP, UDP and port numbers.
  2. Module 1 exam: How networks work · 13 questions · 20 min, without help. 5 topics of this module still to finish first.
  3. Then the next module, Ethernet, MAC addresses and the Cisco CLI. 57 topics left in the course before the final exam.