S4 ICT | Networks and the Internet Lesson Prep Guide (for tutors)

S4 ICT | Networks and the Internet Lesson Prep Guide (for tutors) | Subnets, protocols and security

2026-11-09

Tutor lesson-prep guides · Secondary ICT (DSE)

Most marks in network questions are lost in two places: forgetting to multiply by eight when converting Byte to bit, and forgetting to subtract two from the host count.

Objectives — Students can: (1) work out usable hosts from a /n prefix and explain the minus two; (2) separate LAN from WAN and match common services to ports; (3) compute transmission time as file bits divided by bandwidth with consistent units, scoring at least 11 of 14 in the diagnostic.

01 Prerequisites (3-minute check)

If any of these fails, fix it before the lesson starts.

Skill	Quick check	Pass mark
Powers of two	What is 2 to the power 5, and 2 to the power 8?	32; 256
Unit conversion	How many bits in a Byte? How many KB in a MB?	8 bits; 1024 KB
Simple division	What is 2000 divided by 100?	20

02 Lesson flow (60 minutes)

Time	Stage	Tutor does	Student does
0-6 min	Draw the LAN boundary	Use a room, a school and the internet as three nested frames	Say where the boundary is
6-18 min	IP and subnets	Model 192.168.1.0/24: first and last addresses unusable, so 254 remain	Work out /27 and /28
18-32 min	Protocols and ports	Four TCP/IP layers; HTTP 80, HTTPS 443, DNS 53, SMTP 25	Match five services
32-44 min	Transmission time	250 MB on 100 Mbps: 250 times 8 is 2000 Mbit, divided by 100 is 20 seconds	Repeat for 10 Mbps
44-56 min	14-question diagnostic	Four question types, marked on the spot: 11 right, 3 wrong	Independent work
56-60 min	Exit check	Three oral questions; any miss points to the stage to reteach	Oral answers

03 Teaching script (read it out)

Scene 1: Boundaries before hardware

A LAN is a network inside one administrative scope managed by one group; a WAN joins several LANs, and the internet is the largest WAN. Students often think Wi-Fi means WAN, but Wi-Fi is only a wireless link and a home Wi-Fi is still that household LAN.
Teaching move: draw three nested boxes (room, school, internet) and place each device while talking, so the boundary is visible.

Students do: place a router, a phone, a school server and cloud storage into a box and justify each choice.

Scene 2: Subnets: one formula, two exceptions

IPv4 is 32 bits, so the address space is 2 to the 32, about 4,294,967,296. A /n prefix uses n bits for the network, leaving 8 minus n host bits. A /24 has 256 addresses, but the network address and the broadcast address cannot be assigned, so 254 are usable. A /27 leaves 5 bits, giving 32 addresses and 30 usable; a /28 gives 14.

Students explain: derive usable hosts for /26 and state why two are removed (answer 62).

Scene 3: Transmission time: units first

Bandwidth is in bits per second while files are quoted in MB, so multiply by eight first: 250 MB is 2000 Mbit. Dividing by 100 Mbps gives 20 seconds, and the same file on 10 Mbps takes 200 seconds. If the question asks about latency, multiply one-way delay by the number of handshakes: 20 ms round trip over four handshakes is 80 ms.

Students do: compute 500 MB on 50 Mbps and narrate the unit at every step.

Scene 4: Security: four different jobs

HTTPS encrypts traffic with TLS and proves the server identity with a certificate, so logins belong on https. A firewall filters traffic by rules and denies closed ports by default, updates patch known holes, and phishing is stopped by a human rule: do not open unknown links. Encryption, filtering, patching and awareness are four separate layers.

Students do: match four scenarios (bank login on public Wi-Fi, fake delivery SMS, unpatched router, open shared folder) to one defence each.

04 Question types and worked steps

Type	Example	Steps	Answer
Usable hosts	192.168.1.0/27	Two to the power of (32 minus 27), minus 2	30 hosts
Transmission time	250 MB file on 100 Mbps	MB times 8 gives Mbit, then divide by bandwidth	20 seconds
Protocol match	HTTPS, DNS, SMTP	Recall ports 443, 53, 25	443, 53, 25
Security judgement	Bank login on cafe Wi-Fi	Check HTTPS and avoid sensitive work on public networks	Use HTTPS and avoid public Wi-Fi

Sanity check (teach this): Sanity checks: usable hosts is always below the raw power of two; time is inversely proportional to bandwidth (10 Mbps takes ten times longer than 100 Mbps); HTTPS always uses 443; 11 of 14 is 78 percent, not 11 percent.

05 Independent practice (3 questions)

Question	Answer	What to watch
Three subnet items	Usable hosts for /26, /28 and /30	Did they subtract two
Two transmission items	500 MB on 50 Mbps; 2 GB on 100 Mbps	Did they multiply by eight
Three protocol and security items	Port matching, purpose of HTTPS, firewall rule	Can they give reasons, not just names

06 Common mistakes and what to say

Mistake	Student writes	Say this
Confusing bit and Byte	250 MB divided by 100 gives 2.5 seconds	The file is in Byte and bandwidth in bit, so multiply by eight first to get 2000 Mbit, then divide.
Forgetting minus two	Answering 256 usable hosts for a /24	256 is the total; the network and broadcast addresses are reserved, leaving 254.
Calling Wi-Fi a WAN	Saying any Wi-Fi connection is a WAN	Wi-Fi is a link type; home Wi-Fi is still that household LAN, and a WAN joins LANs together.
Treating HTTPS as total safety	Assuming HTTPS means no malware	HTTPS protects transport and proves identity; phishing sites can hold certificates too, so read the URL.

07 Homework (paper, section and question numbers)

Secondary ICT chapter index — All chapters in one place
Master prep guide index — Compare related chapters
Redo this chapter — Redo diagnostic questions 1 to 7, writing units first

08 Exit check (last 5 minutes)

Question	Answer	Decision
Usable hosts on a /28	14	Answering 16 means minus two was skipped; reteach stage two
Time for 250 MB on 100 Mbps	20 seconds	Answering 2.5 means no times eight; reteach stage three
Which port does HTTPS use	443	Answering 80 means reteach stage two

09 Tutor FAQ

How many ports must be memorised?

Five is enough at this level: HTTP 80, HTTPS 443, FTP 21, SMTP 25 and DNS 53; other questions supply the value.

Must subnet questions use binary?

When the prefix is a multiple of eight you can count bits directly; otherwise two to the power of (32 minus n) is fastest, and binary is only for explaining why.

Should protocol overhead be subtracted?

Unless the question gives an overhead figure, use the ideal value and state the assumption in one sentence.

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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.