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S2 ICT | Computer Networks and the Internet: Tutor Preparation Guide | LAN, WAN, IP, DNS, download time
Students use the internet every day, yet ask why a 60 MB video takes 48 seconds and most answer that the connection is slow. This lesson turns the internet into three numbers a student can write down: how many addresses, how many steps, how many seconds.
Objectives — Students can: (1) separate LAN from WAN by coverage and by who owns and maintains it, and place a school laboratory and a city-wide network correctly; (2) state that IPv4 has 32 bits, giving 2^32 = 4,294,967,296 addresses, and describe a DNS lookup in four steps (local cache, root server, top-level domain server, authoritative server); (3) compute a download time as size times 8 divided by bandwidth, so 60 MB at 10 Mbps is 48 seconds and 1,536 MB at 50 Mbps is 245.76 seconds, about 4 minutes 6 seconds.
01 Prerequisites (3-minute check)
If any of these fails, fix it before the lesson starts.
| Skill | Quick check | Pass mark |
|---|---|---|
| Unit conversion | How many megabits are in one megabyte? | Eight, because a byte has eight bits |
| Powers | What is 2 to the power of 8? | 256, so eight bits can hold 256 different values |
| Reading a speed | What does Mbps stand for? | Megabits per second |
02 Lesson flow (45 minutes)
| Time | Stage | Tutor does | Student does |
|---|---|---|---|
| 0-5 min | Estimate first | Write 60 MB and 10 Mbps on the board and ask for a guess | Most guesses are far too low; the answer is 48 seconds |
| 5-14 min | LAN and WAN | Draw three layers: classroom, school, city, with the router at each boundary | State coverage and ownership for each |
| 14-24 min | IP and DNS | Write 2^32 and walk through one four-step lookup, naming the server asked at each step | Recite the four steps in order |
| 24-34 min | Download time | Start from 60 MB, then 1,536 MB, stressing the times-eight step | Both answers carry the right unit |
| 34-40 min | Four question types | One each on classification, address count, lookup steps and time | All four answered with steps |
| 40-45 min | Exit check | Collect three answers and mark them on the spot | Hand in three answers |
03 Teaching script (read it out)
Scene 1: Act 1: LAN and WAN differ in reach, not in speed
“A LAN is a range you own and can maintain: eight computers and a router in a laboratory. A WAN joins many LANs, such as a city broadband network. Ask two questions to tell them apart: how far does it reach, and who fixes it? The laboratory cabling is plugged in by the school, so it is a LAN; the line pulled by an ISP is not yours, so it is a WAN. The usual error is assuming a WAN must be faster. Speed depends on bandwidth, not on size: a home LAN can beat a school WAN. Two sentences are enough for the exam: coverage, and who owns and manages it.”
Ask coverage, then ownership; WAN does not mean faster.
Scene 2: Act 2: addresses are finite, DNS is the phone book
“IPv4 uses 32 bits, so the total is 2 to the power of 32, which is 4,294,967,296 addresses. The world has more devices than that, so we share addresses with NAT or move to IPv6, which uses 128 bits. Remember one thing: computers know addresses, people know names. You type school.edu.hk, so the computer asks DNS. One, is it in the local cache? Two, if not, ask a root server. Three, the root points to the top-level domain server for .hk. Four, that server points to the authoritative server for the domain, which returns the IP. If the cache already holds the answer, one step is enough, which is why a second visit to the same site feels quicker.”
Finite addresses need sharing; DNS is four steps, cache shortens it.
Scene 3: Act 3: download time has exactly one formula
“Size is in bytes and bandwidth is in bits, so multiply by eight first. 60 MB is 480 Mbit; at 10 Mbps that is 480 divided by 10, which is 48 seconds. On a 50 Mbps line with a 1,536 MB file: 1,536 times 8 is 12,288 Mbit, divided by 50 is 245.76 seconds, or 4 minutes 6 seconds. Two checks: the answer is in seconds, not megabytes, and doubling the file must double the time. One honest caveat: this is a theoretical minimum, so add latency and other users sharing the line, and write at least before the number.”
Multiply by eight, divide by bandwidth, and write at least.
04 Question types and worked steps
| Type | Example | Steps | Answer |
|---|---|---|---|
| Classifying a network | Ten computers and one printer in a school library: LAN or WAN? Give two reasons. | (1) Ask the coverage (2) ask who owns and maintains it (3) conclude (4) give a contrasting example | A LAN: one room, and the school owns and maintains the cabling and devices |
| Counting addresses | How many IPv4 addresses exist, and why move to IPv6? | (1) State 32 bits (2) write 2^32 (3) compute 4,294,967,296 (4) note devices exceed it | 2^32 = 4,294,967,296; IPv6 uses 128 bits, far beyond demand |
| Lookup steps | What happens on a DNS lookup for a never-visited domain? | (1) Local cache (2) root server (3) top-level domain server (4) authoritative server | Cache, root, .hk TLD, authoritative server, then the IP is returned |
| Download time | How fast can a 1,536 MB video arrive on a 50 Mbps line? | (1) Times eight for megabits (2) divide by bandwidth (3) convert to minutes (4) add at least | 12,288 divided by 50 is 245.76 seconds, about 4 minutes 6 seconds at best |
Sanity check (teach this): Sanity check: the IPv4 count must be the ten-digit 4,294,967,296, better read as about 4.3 billion; a download time must be eight times larger than size divided by bandwidth, so six seconds for 60 MB means the times-eight step was skipped; 245.76 seconds must exceed 240 seconds, so four minutes six seconds and 245.76 seconds agree; doubling the bandwidth must halve the time, which is the quickest self-check available.
05 Independent practice (3 questions)
| Question | Answer | What to watch |
|---|---|---|
| How long does a 240 MB update take on a 20 Mbps line? | 240 times 8 is 1,920 Mbit; 1,920 divided by 20 is 96 seconds | Multiply by eight, then divide |
| A phone is on Wi-Fi and mobile data at once; who gives it an IP? | The DHCP server on each network, usually the router, so the phone holds two addresses | Name DHCP and the router |
| A site loads faster the second time. Which DNS step explains it? | The local cache already holds the record, so the root and TLD steps are skipped | Say cache, not bandwidth |
06 Common mistakes and what to say
| Mistake | Student writes | Say this |
|---|---|---|
| Treating WAN as the fast network | “WAN means wide, so it must be the quick one.” | “WAN describes reach, not speed. A laboratory LAN on gigabit cabling can beat a home WAN; bandwidth decides speed.” |
| Skipping the times-eight step | “60 MB divided by 10 Mbps is six seconds.” | “Megabytes and megabits differ by eight: 60 MB is 480 Mbit, so 48 seconds. Multiply by eight first, every time.” |
| Misplacing the digits of the IPv4 count | “IPv4 has 32 addresses, because it is 32 bits.” | “32 is the bit count, not the number of addresses. 2 to the power of 32 is 4,294,967,296, about 4.3 billion.” |
| Believing DNS is the connection | “DNS is the internet; no DNS, no web.” | “DNS only turns a name into an address. Typing the IP works too; people simply cannot remember numbers.” |
07 Homework (paper, section and question numbers)
- Chapter practice: classify three scenarios as LAN or WAN with two reasons each — Networks chapter, practice question 1
- Chapter practice: compute two download times on different bandwidths, each written with at least — Networks chapter, practice question 2
- Extension: open the home router page and copy the DHCP address and gateway it gave your computer — Networks chapter, practice question 3
08 Exit check (last 5 minutes)
| Question | Answer | Decision |
|---|---|---|
| Which two questions separate LAN from WAN? | Coverage, and who owns and maintains it | Wrong means reteach Act 1 |
| How long is 60 MB at 10 Mbps? | 48 seconds, since 480 divided by 10 | Answering six seconds means Act 3 again |
| Name the four DNS steps. | Local cache, root server, TLD server, authoritative server | A missing step means redraw the lookup |
09 Tutor FAQ
Should subnet masks be taught here?
No. Secure LAN, WAN, IP and DNS first; subnetting belongs in senior years.
Do 5G and Wi-Fi 6 belong in the lesson?
One sentence as an example is fine, but the marks still come from reach and ownership, bandwidth and time.
Why does real speed vary?
Shared bandwidth: more users on the same line means less each, which is why answers say at least.
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A TutorZone lesson-prep guide. Written by the TutorZone team; questions and lesson flow are original and are not taken from any school or publisher material.