DSE ICT | Data communications and networking: a tutor preparation guide (packet switching, IP addressing, bandwidth calculations)

2026-11-10

Tutor lesson-prep guides · Secondary ICT

The prose in this chapter reads easily; the arithmetic falls apart. Megabits become megabytes, bits become bytes, and an IP address gets called a hardware address. All three come from one habit: units and layers are kept in the head instead of on paper. This lesson pins them down with one three-layer table and a fixed three-step calculation.

Objectives — Students can: (1) explain packet switching against circuit switching using the presence or absence of a reserved path, with one suitable use for each; (2) classify an IPv4 address and decide whether two addresses share a subnet; (3) compute a file transfer time in three steps, converting units first, and explain why the real time is longer than the theoretical one.

01 Prerequisites (3-minute check)

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

Skill	Quick check	Pass mark
Bits and bytes	How many bits are in one byte	8 bits
Binary	What is binary 1100 in decimal	12
Rate notation	In 20 Mbps, what does the lower-case b mean	Bit per second, not byte

02 Lesson flow (60 minutes)

Time	Stage	Tutor does	Student does
0-6 min	Opening contrast	Send the same video two ways; ask where the difference lies	Write down one difference
6-18 min	Packet switching	Model split, label, route independently, reassemble	Draw the split and the reassembly
18-30 min	IP and subnets	Four octets, network and host parts, subnet mask	Decide whether two hosts share a network
30-44 min	Units and time	Bits and bytes, the two meanings of M, the three steps	Compute one item and state the unit
44-56 min	Four question types	Walk one concept, one IP, one calculation and one scenario question	Do one alone and self-mark
56-60 min	Exit check	Collect two questions; mark whether units or layers slipped	Work independently

03 Teaching script (read it out)

Scene 1: Packet switching: the point is that no path is reserved

Students can recite that packets are cut up, yet cannot state the decisive difference. Give one test: is a dedicated path reserved before communication? Circuit switching yes, like a telephone call; packet switching no, like a parcel service. Derive three consequences from that test: links are used more efficiently, packets may take different routes and therefore arrive out of order, and reassembly plus retransmission are needed. Write the four moves in order: split, attach a header with a sequence number and destination, route independently, reassemble at the destination. Students who put reassembly before routing are reciting, not reasoning. Close with a choice: a video call or a file download, which suffers more from delay? The call, because it must be continuous, and that turns the criterion into a decision tool.

Students say: one sentence on how the two switching methods differ, with one scenario each.

Scene 2: IP addresses: separate the layers before you judge

Students treat an IP address as a computer identity card and then cannot separate it from a MAC address. Two layers settle it: IP is a logical address that changes with the network, MAC is a hardware address burned into the adapter. Deciding a shared subnet takes two steps: align the subnet mask against all four octets, then compare only the network part; a different host part still means the same network. Model it by writing the two addresses on consecutive lines, aligning octet under octet, and underlining only the network part, so nobody compares the wrong field. The three numbers 32 bits, four octets and 0 to 255 must come from the students, because the address-range question is built on exactly those.

Students do: decide whether two addresses share a /24 network and say which octet holds the network part.

Scene 3: Transmission time: convert, multiply by eight, then divide

Nearly every error happens in step one. Fix three steps on paper: (1) turn the file size into bits (multiply MB by 8, then by 1024 twice, or by 1000 twice when the question defines M that way); (2) turn the bandwidth into bits per second; (3) divide and write the unit seconds. Worked case: a 12 MiB file over a 48 Mbps link takes 2.10 seconds. A student who reads 48 Mbps as 48 MB/s gets about 0.26 seconds, eight times too small, which a magnitude check catches immediately. After step two, always add one sentence: the real time is longer because of headers, acknowledgements and retransmissions, and that sentence is where the scenario marks live. Then show the decimal variant: with 12 MB and 48 Mbps the first step becomes 12 x 1000 x 1000 and the answer changes, so the stated unit matters more than the algebra.

Students do: compute one item in three steps and write down why the real time is longer.

04 Question types and worked steps

Type	Example	Steps	Answer
Comparison	Packet switching against circuit switching	One criterion plus three consequences	The criterion must mention a reserved path
IP judgement	Do two addresses share a subnet	Align four octets, compare the network part only	Show the mask and the comparison
Calculation	Find the transfer time	To bits, to bits per second, divide	Unit written and magnitude sensible
Scenario	Which network arrangement suits an application	Delay-sensitive or throughput-sensitive first, then choose	The choice must follow the reason

Sanity check (teach this): One criterion, a reserved path or not, carries the whole comparison question instead of four memorised sentences. IP questions always align four octets first, because comparing the wrong octet is the main loss. The three calculation steps are fixed and the unit is compulsory, since every factor-of-eight error traces back to bits and bytes. Each calculation is followed by a magnitude check.

05 Independent practice (3 questions)

Question	Answer	What to watch
One criterion	State the difference and one scenario for each method	Check whether it reduces to one being faster
Same network	Decide two addresses under /24	Check whether the wrong octet was compared
Transfer time	12 MiB over 48 Mbps	Check the conversion and the magnitude

06 Common mistakes and what to say

Mistake	Student writes	Say this
Reading Mbps as MB/s	Answering 0.26 s for 12 MiB over 48 Mbps	“That b is a bit. Did you multiply by eight? Look at the magnitude before you write it down.”
Forgetting the factor of eight	Dividing megabytes by megabits	“The file is in bytes and the link is in bits. You cannot divide across units.”
Calling IP a hardware address	Saying an IP is burned into the adapter	“When the machine moves to another school, does its IP change? Then it is logical; MAC is the burned one.”
Ignoring overhead	Equating real time with theoretical time	“Where do headers, acknowledgements and retransmissions go? Real time is longer, and saying so earns the mark.”

07 Homework (paper, section and question numbers)

Secondary ICT chapter index — All chapters for this subject
Tutor guide master index — Matching chapters in other subjects
Redo this chapter — Practice items 2, 4 and 6: three-step calculation and subnet check

08 Exit check (last 5 minutes)

Question	Answer	Decision
One criterion	Whether a dedicated path is reserved	Cannot state it sends you back to act one
Two subnet steps	Align four octets, compare the network part	“First two octets match” sends you back to act two
Three calculation steps	To bits, to bits per second, divide	A missing unit sends you back to act three

09 Tutor FAQ

Should subnet bit operations be taught?

DSE asks for concepts and simple judgements; prefix notation such as /24 is enough, with bitwise work as extension.

Is M 1000 or 1024?

Follow the definition given in the question, teach both, and require the choice to be written beside the working.

Are the drawbacks of packet switching needed?

Yes, variable delay and reassembly cost, because scenario questions are built on them.

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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.