Nothing travels as one piece
When you send a photo, it does not sail across the internet intact. It is chopped into packets of a few hundred bytes, each stamped with its destination and its position in the sequence. They may take different routes and arrive out of order.
This sounds fragile and is the opposite. If one packet is lost, only that packet is resent — not the whole photo. If a cable is cut, later packets simply route around it. The network survives damage precisely because it never depends on a single path.
Why not just send it whole?
A single long transmission would monopolise every link along its route until it finished, and any error would mean starting again from the beginning. Packet switching lets thousands of conversations interleave on the same wire and makes recovery cheap.
Layers: each one lies to the one above it
Networking is built in layers, and the rule is that each layer offers the one above a simple, comfortable illusion while hiding the mess underneath.
Your browser believes it has a clean, ordered, reliable stream of bytes to a server. It does not. Underneath, TCP is renumbering packets and requesting retransmissions, IP is picking routes hop by hop, and the physical layer is pushing voltages down copper. Each layer can be replaced — wifi for ethernet, say — without any layer above noticing.
| Layer | Job | You would recognise |
|---|---|---|
| Application | What the message means | HTTP, DNS, SMTP |
| Transport | Getting it there in order, or fast | TCP, UDP |
| Network | Choosing a route across networks | IP, routers |
| Data link | Moving a frame across one hop | Ethernet, MAC addresses |
| Physical | Voltages, light, radio | Cables, wifi, fibre |
Addresses, and the two kinds you have
Your device has a MAC address burned into its network card — permanent, and only meaningful on your local network. It also has an IP address, assigned by whichever network you have joined, which changes when you move.
The analogy that actually holds: the MAC address is like your name, fixed and unique; the IP address is like your current street address, which tells the postal system where to deliver. Routing needs the address, not the name.
Watch packets travel from the source node to the destination. Click Cut link to break the shortest path and watch the traffic reroute through a longer one without stopping. That resilience is the entire design goal of the internet.
TCP or UDP: reliability versus speed
TCP guarantees delivery. It numbers packets, acknowledges each one, retransmits anything lost and reassembles them in order. This costs round trips, which cost time.
UDP guarantees nothing. It fires packets off and never looks back. For a video call that is the right choice — a packet arriving 400 ms late is useless anyway, and you would rather have a brief glitch than a stall. Downloading a file, you would rather wait than lose a byte.
| TCP | UDP | |
|---|---|---|
| Delivery | Guaranteed | Best effort |
| Ordering | Reassembled in order | Whatever arrives |
| Setup | Three-way handshake | None |
| Overhead | Higher | Minimal |
| Used for | Web, email, file transfer | Video calls, live streaming, DNS, games |
What actually happens when you type a URL
Worth walking through once, because it exercises every layer at the same time and is a favourite exam question.
- DNS lookup — the name stemlinepk.com is not an address, so your machine asks a DNS server to translate it into an IP address.
- TCP handshake — SYN, SYN-ACK, ACK. Three messages establish a connection before any real data moves.
- TLS handshake — for https, both sides agree on encryption keys so nobody in between can read the traffic.
- HTTP request — your browser asks for the page. The server answers with a status code and the content.
- Rendering — the HTML arrives, references more files, and the whole cycle repeats for each of them.
Networks by size, and the hardware that joins them
Networks are classified by how much ground they cover, and the vocabulary is examined directly. A LAN (local area network) covers one site — a school, an office, a home — and the organisation owns the cabling. A WAN (wide area network) spans towns or countries and uses infrastructure rented from telecommunications providers; the internet is the largest WAN there is. A MAN sits between the two, covering a city, and a PAN covers a few metres — a phone paired with headphones.
The devices that build these networks each do one job, and questions ask you to distinguish them.
| Device | What it does |
|---|---|
| Switch | joins devices within one LAN and sends each frame only to its destination |
| Router | joins different networks together and chooses the path between them |
| Modem | converts between the computer's digital signal and the line's analogue one |
| Access point | provides wireless connection to a wired network |
| Firewall | filters traffic entering or leaving, blocking what is not permitted |
| Server | a computer providing a service — files, web pages, email — to clients |
Switch or router?
A switch works within one network; a router works between networks. The box in a home labelled "router" is usually all four devices in one case — modem, router, switch and wireless access point — which is why the distinction blurs in everyday use and is still tested carefully in the exam.
Topologies, and wired against wireless
The topology is the shape in which devices are connected. In a star, every device has its own link to a central switch: one failed cable affects one machine, but the switch is a single point of failure. In a bus, all devices share one cable, which is cheap and now rare because a single break stops everything. In a mesh, devices connect to several others, so there are multiple paths and the network survives failures — expensive, and the design the internet itself uses.
The choice between cable and wireless is a separate decision. Cable is faster, more reliable and harder to intercept. Wireless requires no installation, allows movement, and is easier to add to a building — but it is shared between users, weakens with distance and walls, and can be received by anyone in range, which is why encryption is not optional.
A school is networking a new computer laboratory of 30 machines. Recommend a topology and a connection type, with reasons.
- Recommend a star topology with a central switch.Each machine has its own cable, so one fault affects one machine and the faulty one is obvious.
- Recommend wired connections for the fixed machines.Thirty machines sharing one wireless channel would be far slower than thirty separate cables, and the computers do not move.
- Add a wireless access point for staff laptops and tablets.Devices that do move need wireless; that is a different requirement from the fixed lab machines.
- Note that the switch is a single point of failure and should be a spare-able item.Acknowledging the weakness of your own recommendation is usually worth a mark.
A wired star topology for the fixed machines, plus an access point for mobile devices.