OSI and TCP/IP Models

Lesson plan

Level

Beginner

Time

About 60 minutes; the steps below add up to 59

Learners list the jobs involved in delivering a web page and see that a browser need not know whether the laptop uses Wi-Fi, which gives them the idea of layers before it is named. The post comparison carries the idea through. They then match jobs to layers before the worked web request, try merging OSI layers before seeing TCP/IP, predict what a router reads, and work out how a large file crosses a small MTU. The habit to build: place any problem or control at a layer, and troubleshoot from the bottom up.

Objectives

• Explain why networking is divided into layers, and why that helps security work
• Name the seven OSI layers in order and the question each answers
• Map OSI onto the four-layer TCP/IP model
• Explain encapsulation, segments, packets and frames, and what an MTU limits

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? lesson first.
• An envelope, a bag and a box make the post comparison physical: wrap a note in each in turn.
• The lesson takes about 59 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

   • Explain why networking is divided into layers, and why that helps security work
   • Name the seven OSI layers in order and the question each answers
   • Map OSI onto the four-layer TCP/IP model
   • Explain encapsulation, segments, packets and frames, and what an MTU limits

   Purpose: Learners list the jobs involved in delivering a web page and see that a browser need not know whether the laptop uses Wi-Fi, which gives them the idea of layers before it is named. The post comparison carries the idea through. They then match jobs to layers before the worked web request, try merging OSI layers before seeing TCP/IP, predict what a router reads, and work out how a large file crosses a small MTU. The habit to build: place any problem or control at a layer, and troubleshoot from the bottom up.

2. 1. Dozens of jobs Think 6 min

   Purpose: Find the need for layers from the jobs involved in delivering a web page.

   Ask

   • List the separate jobs needed to get a web page to your phone.
   • Does the browser need to know about Wi-Fi?

   Responses to expect

   • Lists usually include finding the server and sending signals; checking arrival is often missed.
   • On screen in turns: the opening question, its answer; then the question on Wi-Fi or cable, and its answer.

   Discussion pause: Collect the jobs on the board before revealing the explanation.

   In the lesson window

   • Think it through: Think about everything that has to happen for a web page to reach your phone from a server in another country. List as many separate jobs as you can.
   • Think it through: A browser asks for a web page. Does the browser need to know whether the laptop is on Wi-Fi or a cable?

3. 2. Why networking is divided into layers Explain 6 min

   Purpose: State the purpose of layers, and use the post as the comparison, with its limit.

   Ask

   • In the post comparison, what does each step add, and what does it ignore?

   Responses to expect

   • Each adds only what the next carrier needs.
   • On screen in turns: the opening; then the cybersecurity value of layers; the same idea in the post, starting with the letter; the envelope; the sack; the van, and how each step wraps the last; where the post comparison stops.

4. 3. The seven OSI layers Explain 14 min

   Purpose: Teach the seven layers by the question each answers, the mnemonic, the reference table and the web request.

   Ask

   • Which layer is where routers work?
   • Match five jobs to their layers before reading the web request.

   Responses to expect

   • Layer 2 and Layer 3 are the most often swapped; switches against routers helps.
   • On screen in turns: the opening; then the seven layers, starting with Layer 7; Layer 6, Presentation; Layer 5, Session; Layer 4, Transport; Layer 3, Network; Layer 2, Data Link; Layer 1, Physical; a sentence to remember the order; the check question on where routers work, and its answer; the layers as a reference, with their protocols; a model for thinking, and typical examples; applying the model: the question on who does what, and its answer; a secure page, from the top layer down.

   Misconceptions to challenge

   • A learner may believe: “Every protocol belongs to exactly one layer.” Respond: OSI is a model; Ethernet spans two layers.

   Discussion pause: The layers reveal one at a time from Layer 7 down: before each, ask the class what question that layer answers.

   If the explanation does not land: Have learners invent their own mnemonic from Layer 1 upwards.

   To go further

   • Where in the seven layers would a firewall that filters by port number work?

   In the lesson window

   • Check your understanding: Which layer answers “Which device, on which network, is it for?”, and is where routers work?
   • Think it through: Before reading on: using the table of questions, which layer would handle each of these for a secure web page? Writing the request; getting it to the right program; addressing the right computer; reaching the next device; sending the signals.

5. 4. The TCP/IP model Predict 5 min

   Purpose: Map OSI onto the four-layer TCP/IP model.

   Ask

   • Which OSI layers would you merge?

   Responses to expect

   • Most merge 5, 6 and 7 first.
   • On screen in turns: the opening question, its answer; then the two models lined up; why the TCP/IP model matters.

   Misconceptions to challenge

   • A learner may believe: “The internet runs on the OSI model.” Respond: It runs on the TCP/IP suite; OSI is a reference model.
   • A learner may believe: “OSI and TCP/IP describe different networks.” Respond: They describe the same work in different amounts of detail; the internet is built on the TCP/IP protocol suite.

   In the lesson window

   • Think it through: OSI has seven layers. If you had to merge some of them to make a simpler model, which would you combine?

6. 5. Encapsulation: wrapping data layer by layer Predict 9 min

   Purpose: Name segment, packet and frame, and follow one request down and back up.

   Ask

   • What does a router halfway across read?

   Responses to expect

   • Some say the whole request; ask whether a sorting office opens letters.
   • Diagram: step through the request down the laptop’s layers, across the routers and up the server’s; ask what is added or removed at each step before it appears.
   • On screen in turns: the opening; then segment, packet and frame, with the diagram; the question on what a router reads, and its answer; one request down and back up the stack, step by step; the diagram’s stages one at a time.

   Misconceptions to challenge

   • A learner may believe: “Every layer adds its own header.” Respond: Only the layers that take part do.

   In the lesson window

   • Think it through: Halfway across the internet, a router receives the wrapped-up web request. What does it need to read to decide where to send it next?

7. 6. MTU: how big can a packet be? Predict 6 min

   Purpose: Explain the MTU and what happens to data larger than it.

   Ask

   • How does a 10-megabyte file cross a 1,500-byte link?

   Responses to expect

   • Splitting is found quickly; who splits it is the new idea.
   • On screen in turns: the opening question, its answer; then when data is bigger than the MTU; troubleshooting from the bottom up.

   In the lesson window

   • Think it through: A link can carry packets of at most 1,500 bytes. How does a 10-megabyte file get across it?

8. 7. 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 no-Wi-Fi question tests bottom-up troubleshooting; the TLS claim tests the model’s limits.
   • On screen in turns: the opening question, its answer; then the question on TLS and Layer 6, and its answer.

   In the lesson window

   • Apply it: A user’s laptop shows no Wi-Fi connection at all. Which layer would you start troubleshooting at?
   • Apply it: A colleague says: “TLS is a Layer 6 protocol, full stop.” What is the best response?

9. 8. Check your understanding Check 6 min

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

   Ask

   • Answer individually, then compare before revealing.

   Responses to expect

   • The model-boundaries question links back to the misunderstandings.
   • The last question, on hosts, nodes and network interface cards, revisits the What Is a Network? lesson. It has more than one correct statement; the statement about the NIC is the one this lesson unlocks: Ethernet and Wi-Fi cover Layers 1 and 2, and the NIC is the hardware that does that work.
   • On screen in turns: the opening; then the revision questions one at a time, each followed by its answer; where to practise next.

10. 9. What to remember Consolidate 3 min

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

    Ask

    • Without looking: say the mnemonic and name each layer.

    Responses to expect

    • Application, Presentation, Session, Transport, Network, Data Link, Physical.
    • On screen in turns: the opening; then the summary; the key terms; the common misunderstandings.

11. 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

    • Each layer does one job and trusts the layer below.
    • Seven OSI layers, from the program down to the signal.
    • Four TCP/IP layers describe the same work more briefly.
    • Data is wrapped on the way down and unwrapped on the way up.
    • Anything bigger than the MTU travels in pieces.

Consolidation

• Each layer does one job and trusts the layer below.
• Seven OSI layers, from the program down to the signal.
• Four TCP/IP layers describe the same work more briefly.
• Data is wrapped on the way down and unwrapped on the way up.
• Anything bigger than the MTU travels in pieces.