Key Idea: Data crosses a network as packets, each carrying an address and routed independently over whatever medium lies between. That independence is exactly what makes the internet survive failures no single designer anticipated.
Paper 1
- Naming devices, protocols and topologies.
- Distinguish questions — router vs switch, LAN vs WAN.
Paper 2
- Scenario design: choose a topology or a medium.
- Transfer-time and subnet calculations.
Both
- Selection questions want a justification against the alternative.
- Say what would go wrong with the option you rejected.
📮 Packet switching
How a message actually travels
- The message is divided into packets, each with a destination address and a sequence number
- Each packet is routed independently and may take a different path
- The receiver reassembles them in sequence order and asks for anything missing
- A router whose queue is full drops packets — that loss is how congestion is signalled back
| Packet switching | Circuit switching | |
|---|---|---|
| Reserves a path? | No | Yes, for the whole conversation |
| Capacity when idle | Used by others | Wasted |
| A link fails | Routed around | The call dies |
| Delay | Varies | Constant |
Every packet carries a fixed-size header, so tiny packets waste a large share of the transmission. But a large packet must be resent entirely when one bit is corrupted, and it holds the link longer, delaying everything queued behind it.
🔢 Addressing
| IPv4 | IPv6 | |
|---|---|---|
| Length | 32 bits | 128 bits |
| Total addresses | ≈ 4.3 billion | ≈ 3.4 × 10³⁸ |
| Status | Exhausted | The replacement |
Three independent questions about any address
- How long is it? IPv4 or IPv6
- Can the internet reach it? Public or private
- Does it change? Static or dynamic
- All three are independent — an address can be private, dynamic and IPv4 at once
The router performs network address translation: outgoing packets leave carrying the router's public address, and it records which internal device each conversation belongs to. When the reply arrives, it looks the conversation up and forwards it to the right private address.
🔌 Transmission media
| Fibre optic | Copper | Wireless | |
|---|---|---|---|
| Carries | Light | Voltage | Radio |
| Bandwidth | Highest | Moderate | Shared, variable |
| Interference | Immune | Picks up noise | Walls, other radios |
| Distance | Tens of km | ≈ 100 m | Short |
| Cost | Highest | Low | No cabling |
Important: A satellite link has huge bandwidth and terrible latency — about a quarter of a second, because the signal travels 36 000 km up and back and nothing exceeds the speed of light. That pair is the clearest proof the two are independent.
🧭 Routing
| Static | Dynamic | |
|---|---|---|
| Routes come from | An administrator, by hand | Routers telling each other |
| A link fails | Traffic keeps going to it | Rerouted automatically |
| Overhead | None | Bandwidth and processing, continuously |
| Suits | One path out | Duplicate links, many sites |
Only the next hop. No router knows the whole path — it looks up the destination, forwards to the next router, and that one does the same. The path is the result, not the plan.
📝 Exam-style questions
A 5 MB file is sent as packets carrying 1250 bytes of data each, with a 50-byte header on every packet. Determine the number of packets and the percentage overhead. Take 1 MB as 1 000 000 bytes.
🔒 Model answer plan
See the mark-by-mark plan — for / against / judgement, with marking guidance — in study mode.
A research station in Antarctica needs internet access. Suggest a transmission medium, justifying it and acknowledging its main drawback.
🔒 Model answer plan
See the mark-by-mark plan — for / against / judgement, with marking guidance — in study mode.
Explain why a router drops packets when its queue is full, rather than storing them until the link is free.
🔒 Model answer plan
See the mark-by-mark plan — for / against / judgement, with marking guidance — in study mode.
✅ Quick check
Cover the answers.
Why do packets carry sequence numbers? They are routed independently and can arrive out of order — the numbers restore the order and reveal which packet is missing.
How many addresses does IPv4 have, roughly? 2³² ≈ 4.3 billion — now exhausted.
Which medium is immune to electrical interference? Fibre optic — it carries light, which electrical noise cannot disturb.
What does a router know about the route? Only the next hop. The full path is the result of each router's independent decision.
Static or dynamic routing for a site with one link out? Static — there is no alternative path to choose between, so a routing protocol would add overhead for nothing.
Exam tips
- Sequence numbers do two jobs: restore the order, and reveal what is missing.
- Every packet carries a fixed header, so tiny packets waste capacity and huge ones waste retransmissions.
- Bandwidth is a rate; latency is a delay. The satellite example proves they are independent.
- A path is only as fast as its slowest link — quote that when comparing routes.
- Usable addresses = 2ʰ − 2, and NAT is why a private address can still browse.
- Dropped packets are how congestion is signalled, not a fault.