Unicast, Multicast, Broadcast and Anycast

Lesson plan

Level

Beginner

Time

About 40 minutes; the steps below add up to 40

Learners work out how a teacher reaches one pupil, one table and the whole room, and how they would find a printer whose location they do not know, which gives them unicast, multicast and broadcast, and the reason for broadcasting, before the terms. They then predict what would happen if broadcasts crossed the internet, which explains why routers keep them local. The habit to build: ask who is meant to receive the traffic, and what the sender knows.

Objectives

• Tell unicast, multicast, broadcast and anycast apart by who is meant to receive the traffic
• Explain what a broadcast domain is and why routers mark its edge
• Explain why broadcasts are kept local, for performance and for security
• Keep delivery method and OSI layer as separate questions

Before the lesson

• Open this page on your own screen and use “Open the lesson window” to open the lesson in a second window. Share only that second window.
• Learners need the What Is a Network? and OSI and TCP/IP Models lessons first.
• The room comparison works well acted out: whisper to one learner, speak to one table, then address everyone.
• The lesson takes about 40 minutes.
• In presentation mode every step is shown in turns: Next reveals one moment, part or answer at a time, and the instructor view names what is on screen and what comes next.

Steps

1. Introduction Opening

   On screen: the lesson’s title, module and number; Next adds its objectives and key terms, as the lesson page lists them; Next again begins the first step.

   Objectives on screen

   • Tell unicast, multicast, broadcast and anycast apart by who is meant to receive the traffic
   • Explain what a broadcast domain is and why routers mark its edge
   • Explain why broadcasts are kept local, for performance and for security
   • Keep delivery method and OSI layer as separate questions

   Purpose: Learners work out how a teacher reaches one pupil, one table and the whole room, and how they would find a printer whose location they do not know, which gives them unicast, multicast and broadcast, and the reason for broadcasting, before the terms. They then predict what would happen if broadcasts crossed the internet, which explains why routers keep them local. The habit to build: ask who is meant to receive the traffic, and what the sender knows.

2. 1. Who is it for? Think 6 min

   Purpose: Find the three delivery choices, and why a device might ask everyone, from familiar situations.

   Ask

   • How does the teacher reach each audience?
   • How would you find the printer?

   Responses to expect

   • Most say “ask around”; connect it to asking everyone at once.
   • On screen in turns: the opening question, its answer; then the question on finding the printer, and its answer.

   Discussion pause: Act out the three ways of speaking before revealing the explanation.

   In the lesson window

   • Think it through: A teacher in a busy room has three things to say: a private word to one pupil, instructions for one table working on a project, and “Pens down, everyone.” How does she reach each audience?
   • Think it through: You need the printer, but you know only its name, not where it is on the network. How could you find it?

3. 2. Who receives the traffic? Explain 12 min

   Purpose: Name unicast, multicast and broadcast, and the broadcast domain, with the limit of the comparison.

   Ask

   • Which method is the device asking for an address using?

   Responses to expect

   • Some say unicast because only one device answers; ask who receives the question.
   • Diagram: before each delivery on the school network, ask the class which devices will receive the message; the broadcast stops at the router.
   • Diagram: before each delivery on the school network, ask the class which devices will receive the message; the broadcast stops at the router.
   • On screen in turns: the opening; then unicast: one to one; multicast: one to a chosen group; broadcast: one to everyone nearby; the broadcast domain; where the comparison with a room stops; two facts to carry forward; the three methods traced on the school network; the diagram’s stages one at a time; the check question on asking for an address, and its answer.

   Misconceptions to challenge

   • A learner may believe: “A broadcast reaches the whole internet.” Respond: Ordinary broadcasts stay within their broadcast domain.

   Discussion pause: Before the broadcast stage, ask whether the router will pass the laptop’s question to the internet.

   In the lesson window

   • Check your understanding: A device that has just joined a network asks every device nearby: “Can anyone here give me an address?” Which delivery method is that?

4. 3. Why broadcasts stay local Predict 5 min

   Purpose: Explain why routers stop broadcasts, and the security side of broadcasting.

   Ask

   • What if What if broadcasts crossed the internet?
   • Why does broadcasting matter to an attacker on the same network?

   Responses to expect

   • The security point is less obvious: any device in the domain sees every broadcast.
   • On screen in turns: the opening question, its answer; then why broadcasts are limited; the security side of broadcasts.

   To go further

   • Which kind of attacker benefits most from seeing every broadcast on a network?

   In the lesson window

   • Think it through: Every device that hears a broadcast has to stop and look at it. What would happen if broadcasts travelled across the whole internet?

5. 4. Anycast Explain 6 min

   Purpose: Introduce anycast and identify the four methods in four cases.

   Ask

   • In the four cases, what does the sender know each time?

   Responses to expect

   • Not knowing whom to ask forces a broadcast; knowing exactly allows unicast.
   • On screen in turns: the opening; then four cases to identify, starting with the laptop’s question; the printer’s reply; the streamed lecture; the DNS query, and what the cases show; checking broadcast volume when a network is slow.

   Misconceptions to challenge

   • A learner may believe: “IPv6 uses broadcast too.” Respond: IPv6 has no broadcast and uses multicast instead.

   If the explanation does not land: A national phone number for a chain of shops that connects you to the nearest branch: one number, many places, and you reach one of them.

6. 5. Apply it in a new situation Apply 4 min

   Purpose: Set up independent application, ideally on a later day.

   Ask

   • Who will try these now, and who later in the week?

   Responses to expect

   • The sluggish network tests broadcast domains; the DHCP claim tests method against layer.
   • On screen in turns: the opening question, its answer; then the question on DHCP and Layer 2, and its answer.

   In the lesson window

   • Apply it: A network grows to 2,000 devices in a single broadcast domain and becomes sluggish. What is a likely cause, and a fix?
   • Apply it: A colleague says: “DHCP uses broadcast, so it must be a Layer 2 protocol.” What is the best response?

7. 6. Check your understanding Check 4 min

   Purpose: Check the lesson against the approved revision-bank questions.

   Ask

   • Answer individually, then compare before revealing.

   Responses to expect

   • The scope and delivery question combines this lesson with What Is a Network?
   • On screen in turns: the opening; then the revision questions one at a time, each followed by its answer; where to practise next.

8. 7. What to remember Consolidate 3 min

   Purpose: Summarise the five points, the key terms and the misunderstandings.

   Ask

   • Without looking: give one real example of each method.

   Responses to expect

   • A web page request, a joined stream, an address request on joining, a nearby DNS server.
   • On screen in turns: the opening; then the summary; the key terms; the common misunderstandings.

9. Lesson recap Closing

   On screen: the lesson’s objectives again; Next adds the closing question, which asks the class to explain each one in their own words without looking back. It says nothing about what anyone has learnt: use the answers to decide what to revisit.

   Points to listen for

   • One, a chosen group, everyone nearby, or the nearest of several.
   • A router is the edge of a broadcast domain.
   • Broadcasts cost every device effort and are visible to every device.
   • What the sender knows decides how it must send.
   • Delivery method and protocol layer are different questions.

Consolidation

• One, a chosen group, everyone nearby, or the nearest of several.
• A router is the edge of a broadcast domain.
• Broadcasts cost every device effort and are visible to every device.
• What the sender knows decides how it must send.
• Delivery method and protocol layer are different questions.