What you'll learn
A network is what turns separate computers into something that can share work, files and conversation. This topic covers how networks are organised, the hardware that joins them, the addresses that identify devices, the protocols that agree how to communicate, the layered model that keeps all those protocols manageable, and the measures that protect data travelling across them. By the end of this guide you should be able to distinguish a LAN from a WAN and client–server from peer-to-peer, explain what each piece of network hardware does, tell a MAC address from an IP address, name the four TCP/IP layers and what each contributes, say why TCP and UDP both exist, and separate bandwidth from latency.
Key terms and definitions
LAN — A local area network covering a single site, where the organisation owns the infrastructure.
WAN — A wide area network spanning separate geographical sites, relying on third-party infrastructure.
Client–server — A model where central servers provide services and resources to client computers.
Peer-to-peer — A model where each computer shares its own resources directly, with no central server.
NIC — The network interface card, the hardware that connects a device to a network.
Switch — A device that forwards data only to the device it is addressed to.
Router — A device that connects networks together and directs traffic between them.
MAC address — A permanent address identifying a device's network hardware.
IP address — An address allocated by the network, used for routing between networks.
Protocol — An agreed set of rules governing how devices communicate.
Encryption — Scrambling data so it cannot be understood if intercepted.
Bandwidth — The amount of data that can be transmitted in a given time.
Latency — The delay between data being sent and beginning to arrive.
Core concepts
LAN and WAN
The distinction is ownership and geography, not size. A LAN covers one site — a school, an office — and the organisation owns the cabling and hardware, so it chooses its own speeds and security. A WAN spans separate sites, and the links between them run over infrastructure belonging to telecommunications companies. A business with three city offices controls each LAN but rents the connections between them, and that is the practical consequence that matters.
Client–server and peer-to-peer
In client–server, central servers hold files, handle printing and manage authentication. Centralising makes backup, security, access rights and software deployment manageable from one place, which is why schools and businesses use it. The cost is a server to buy and a technician to run it.
In peer-to-peer, every computer has equal status and shares directly. There is no server to buy or administer, which suits a home or a very small office. It scales badly: backup falls to each user, and a file is only available while the machine holding it is switched on.
Network hardware
- The NIC connects a device to the network and carries its MAC address. Modern machines have one built into the motherboard.
- A switch reads the destination MAC address and sends data down just that one port, keeping traffic off the rest of the network.
- A router joins networks together — typically a LAN to the internet — and decides where to forward data next.
- Transmission media are the cables or radio links that carry the signal.
Wired and wireless
A cable gives a dedicated path with no interference from walls, neighbours or other devices, so wired Ethernet generally has lower latency and more reliable transmission. Wi-Fi trades that reliability for mobility, which for most users is the more valuable of the two.
Wi-Fi transmits on channels within a frequency band. Two nearby networks on the same channel interfere: each has to wait for the other and transmissions are corrupted more often, so speed and reliability both fall. Moving one network to a different channel is the standard fix.
MAC and IP addressing
A MAC address is set by the manufacturer and identifies the physical hardware, so it stays with the device wherever it goes. An IP address is allocated by the network and changes when the device joins a different one.
On a home network each device has a private IP address meaningful only inside that network, and the router translates between those and its single public address. This is why a web server sees the household rather than the individual laptop that made the request.
Protocols
Each protocol has one clearly defined job:
- HTTP transfers web pages. HTTPS does the same with the data encrypted, so an eavesdropper sees only scrambled text rather than passwords or card numbers.
- FTP transfers files to and from a server.
- SMTP sends email; IMAP retrieves it, keeping the mailbox on the server and synchronising it — which is why the same messages appear on a phone and a laptop. A mail client typically needs both.
- TCP and UDP handle transport, and IP handles addressing and routing.
TCP and UDP
TCP numbers packets and acknowledges them, resending anything lost, so the data arrives complete and in order. That reliability costs delay. UDP does none of this: it accepts the occasional lost packet in exchange for speed.
Which is right depends on whether a late packet is still worth having. For a file transfer it always is, so TCP wins. For live video a retransmitted frame would arrive after the moment it was meant to be shown, so it is worthless even when it turns up — which is why streaming uses UDP and accepts a little visible glitching.
The four-layer TCP/IP model
| Layer | What it does |
|---|---|
| Application | The protocol the program itself speaks (HTTP, SMTP, FTP) |
| Transport | Splits data into packets, numbers them, reassembles them (TCP, UDP) |
| Internet | Adds addressing and routes packets between networks (IP) |
| Link | Handles the physical connection and the local hardware |
Layering exists so that each layer can change independently of the others. A new Wi-Fi standard changes the link layer while browsers and email clients carry on unaltered — the application never needs to know how the bits physically travel. Layering also makes faults easier to isolate, because each layer has a defined job.
Packet switching
Data is divided into packets that may travel by different routes and are reassembled at the far end. The advantage is resilience: if one route is busy or fails, packets travel another way, so no single broken link stops communication. The cost is that packets can arrive out of order or not at all — which is exactly the problem TCP exists to solve.
Bandwidth and latency
Bandwidth is capacity, usually in megabits per second, and it is shared between everyone using the connection. That is why a school network slows at break time although nothing has broken: the same capacity divided among three hundred phones gives each a small fraction.
Latency is the delay before data begins to arrive, and it is independent of bandwidth. A satellite link can carry enormous quantities of data while taking half a second to deliver the first bit. Gaming and voice calls are ruined by high latency even when bandwidth is plentiful.
Network security
- Encryption assumes interception will happen and makes it useless, which matters most on wireless networks where the signal travels through the air.
- Strong passphrases and changing default passwords close the easiest routes in; factory passwords by model are published online.
- MAC address filtering restricts which devices may connect, but gives only limited protection: MAC addresses travel unencrypted and an attacker can copy a permitted one. It is worth having as one obstacle among several, never as a substitute for encryption.
- Firewalls control which connections may cross the network boundary.
Worked examples
Example 1: Why a file arrives complete
A large file arrives intact and in order from the other side of the world. Which protocols made that possible?
IP found a route for each packet but promised nothing about delivery. TCP numbered the packets, acknowledged them and resent anything lost, turning an unreliable path into a complete, ordered file. Both are needed, and they do different jobs — IP routes, TCP guarantees.
Example 2: The break-time slowdown
A school's network crawls at break although its internet bandwidth has not changed.
Bandwidth is a shared capacity. Far more devices are now using the same fixed amount at once, so each gets a smaller share. Nothing is faulty: the network is behaving exactly as a shared resource under heavier demand. Increasing bandwidth or limiting usage would help; replacing hardware would not.
Example 3: A new Wi-Fi standard
A faster Wi-Fi standard is released. What must change, and what must not?
The link layer changes: the network interface hardware, the signal encoding and the frequencies or channels used. Everything above that layer — browsers, email clients, the HTTP and TCP software — carries on unaltered. That separation is exactly the benefit layering buys.
Example 4: Choosing a network model
A business with 60 computers must decide between client–server and peer-to-peer.
Client–server is right. Central backup, access rights managed in one place, and software deployed from a single point are all decisive at that scale. The trade-off is honest: it increases the need for a technician rather than removing it, and that administrative cost is the main argument for peer-to-peer in a very small office.
Common mistakes and how to avoid them
The commonest error is defining a LAN by size. It is defined by ownership of the infrastructure over a single site; a large campus network is still a LAN.
Do not swap MAC and IP. The MAC is fixed in the hardware by the manufacturer; the IP is allocated by the network and changes when the device moves.
Keep bandwidth and latency separate. Bandwidth is how much; latency is how soon. A connection can have plenty of one and be ruined by the other.
With TCP and UDP, the difference is reliability versus speed, not encryption and not wired versus wireless. TCP checks and resends; UDP does not.
Do not say the layered model makes the network faster. It makes the system manageable: layers can be developed independently and faults isolated.
Remember that MAC filtering is weak on its own, because addresses can be observed and copied.
Finally, when asked what a switch does, say it forwards data to the addressed device. Joining a network to the internet is the router's job, and storing shared files is a server's.
Exam technique for "Fundamentals of Computer Networks"
For protocol questions, give the full name and the one job it does. Examiners want "SMTP sends email" rather than "SMTP is for email stuff", and a question offering four protocols is testing whether you can separate the sending one from the retrieving one.
When asked about the layered model, name the layer and what it contributes. If the question is about an advantage, say that layers can be changed independently and give a concrete example.
Scenario questions about slow or unreliable networks almost always want you to identify a shared or contended resource — bandwidth divided among many users, or two networks on one Wi-Fi channel. Name the resource, say why it is contended, then propose a fix.
For security questions, give the measure and its limit. Full marks usually need both: "MAC filtering restricts which devices may join, but addresses can be copied, so encryption is still required."
Quick revision summary
- LAN = one site, own infrastructure. WAN = separate sites, third-party links.
- Client–server centralises backup, rights and deployment; peer-to-peer needs no server but scales badly.
- NIC connects; switch forwards to the addressed device; router joins networks.
- MAC = permanent hardware address. IP = allocated by the network, changes when the device moves.
- HTTPS = HTTP with encryption. FTP = files. SMTP = send mail. IMAP = retrieve mail.
- TCP numbers and resends packets (reliable, slower); UDP does not (fast, suits live streaming).
- The four layers are application, transport, internet, link — each changeable independently.
- Packet switching gives resilience; packets may arrive out of order, which TCP fixes.
- Bandwidth is shared capacity; latency is delay. They are independent.
- Security: encryption, strong passphrases, changed default passwords, firewalls; MAC filtering is weak alone.