Talib AliCybersecurity Study Guide & Portfolio

Study Guide Instructor view

Classful Addressing, Subnet Masks and CIDR

Teaching notes for Lesson 3.6, with controls for presenting it.

Keep this page on your own screen. Share only the lesson window. The lesson window holds no teaching notes, so sharing it cannot show them.

Presenting

  1. Open the lesson in its own window and share that window, not this one.
  2. Use Next and Previous here, or the lesson’s own, to move through the lesson. Next reveals one thing at a time: the next idea, or the answer to the question on screen, then the next step. Previous takes the last reveal back. The line below says what is on screen and what Next will show.
  3. Reveal hints and answers when the class is ready, and reset a demonstration before using it again.

The controls need JavaScript and a browser that supports BroadcastChannel. Without them, open the lesson in presentation mode and use its own controls.

The two windows talk to each other only inside this browser, on this device. Nothing is sent to a server, and no account is needed. A second device (a phone or another tablet) cannot follow the lesson window yet.

Lesson plan

Level
Beginner
Time
About 45 minutes; the steps below add up to 46

Learners work out what a laptop must decide before sending to a neighbour and to a distant server, and what one extra host bit does, so the purpose of the boundary comes before any mask. They then invent a way to mark network bits with a second 32-bit number before the subnet mask is named, and a shorter way to write it before CIDR. The habit to build: count the 1 bits, and treat the prefix as configured, not implied.

Objectives

  • Explain why a device needs to know where the network portion ends
  • Describe historical classful addressing and why it was replaced
  • Read a subnet mask as a run of 1 bits followed by 0 bits
  • Convert between a subnet mask and a CIDR prefix
  • Apply a configured prefix regardless of an address’s historical class

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 IPv4 and IPv6 Addresses and Number Systems lessons first.
  • Keep the nine mask-octet values on the board: 0, 128, 192, 224, 240, 248, 252, 254, 255.
  • The lesson takes about 46 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 a device needs to know where the network portion ends
    • Describe historical classful addressing and why it was replaced
    • Read a subnet mask as a run of 1 bits followed by 0 bits
    • Convert between a subnet mask and a CIDR prefix
    • Apply a configured prefix regardless of an address’s historical class

    Purpose: Learners work out what a laptop must decide before sending to a neighbour and to a distant server, and what one extra host bit does, so the purpose of the boundary comes before any mask. They then invent a way to mark network bits with a second 32-bit number before the subnet mask is named, and a shorter way to write it before CIDR. The habit to build: count the 1 bits, and treat the prefix as configured, not implied.

  2. 1. My network or another? Think 6 min

    Purpose: Find why a device needs the boundary, and what host bits do.

    Ask

    • What must the laptop decide before sending each one?
    • One more host bit: what happens to the number of hosts?

    Responses to expect

    • “Is it local?” comes quickly once asked which device joins networks.
    • On screen in turns: the opening question, its answer; then the question on one more host bit, and its answer.

    Discussion pause: Let learners suggest how the laptop could tell before revealing it.

    In the lesson window

    • Think it through: A laptop at 192.168.1.10 wants to send something to 192.168.1.77, and later something to 203.0.113.5. What must it decide before sending each one?
    • Think it through: Suppose the network portion of an address is made one bit shorter, giving the host portion one more bit. What happens to the number of possible host addresses?

  3. 2. Why a boundary is needed Explain 3 min

    Purpose: State the two jobs of the boundary.

    Ask

    • Which two things does the boundary decide?

    Responses to expect

    • Local or remote, and how many hosts.
    • On screen in turns: the opening; then how the boundary also decides a network’s size.

  4. 3. Historical classful addressing Explain 6 min

    Purpose: Describe the historical classes and why they were wasteful.

    Ask

    • What decided the class?

    Responses to expect

    • Some think the mask did; it was implied by the first octet.
    • On screen in turns: the opening; then the historical classes; the check question on what decided the class, and its answer; whether both octets of a Class B address mattered; leading bits, waste, and the end of classes.

    Misconceptions to challenge

    • A learner may believe: “An address’s class decides its mask.” Respond: Historically yes; under CIDR the prefix is configured.

    If the explanation does not land: Like shoe shops that sold only sizes 4, 8 and 12: most people got a size far too big. CIDR is shoes made to measure.

    In the lesson window

    • Check your understanding: Under the historical scheme, what decided which class an address belonged to?

  5. 4. Subnet masks Predict 5 min

    Purpose: Introduce the subnet mask as a run of 1s followed by 0s.

    Ask

    • How could another 32-bit number mark the network bits?

    Responses to expect

    • Most reach “1 for network, 0 for host” with one hint.
    • On screen in turns: the opening question, its answer; then what a subnet mask is, with 255.255.255.0; the nine values a mask octet can take.

    Misconceptions to challenge

    • A learner may believe: “A mask can mix 1s and 0s in any order.” Respond: A valid mask is one unbroken run of 1s, then 0s.

    In the lesson window

    • Think it through: Suppose you had to mark which of an address’s 32 bits belong to the network part, using another 32-bit number. How could you do it?

  6. 5. CIDR notation Predict 10 min

    Purpose: Introduce CIDR and convert between masks and prefixes.

    Ask

    • What shorter way could write 255.255.255.0?
    • What prefix is 255.255.255.224?

    Responses to expect

    • /26 is the usual slip for 224; count the bits.
    • On screen in turns: the opening question, its answer; then masks and prefixes, starting with /8; /16; /24; /20; /26; what the last two rows show; converting a mask to a prefix; converting a prefix to a mask; the check question on 255.255.255.224, and its answer.

    Misconceptions to challenge

    • A learner may believe: “/24 and 255.255.255.0 are different settings.” Respond: They are one boundary written two ways.

    Discussion pause: Before each row of the mask table appears, ask the class to count the 1 bits and give the prefix.

    To go further

    • How many host addresses does a /20 allow, before reserving any?

    In the lesson window

    • Think it through: The mask 255.255.255.0 is twenty-four 1 bits followed by eight 0 bits. What shorter way could you write the same boundary?
    • Check your understanding: What is the prefix length for the mask 255.255.255.224?

  7. 6. Network and host identifiers under CIDR Predict 6 min

    Purpose: Show that the configured prefix overrides the old class, and find the network and host identifiers.

    Ask

    • Can 131.107.5.15 be a /24?

    Responses to expect

    • Some say no because it was Class B.
    • On screen in turns: the opening question, its answer; then 131.107.5.15 with a /24; network and host identifiers; masks and prefixes on real systems.

    In the lesson window

    • Think it through: The address 131.107.5.15 starts with 131, which was historically Class B and so implied /16. Can it be given a /24 today?

  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 /22 question tests conversion; the broken mask tests the unbroken-run rule.
    • On screen in turns: the opening question, its answer; then the question on a mask of 255.255.0.255, and its answer.

    In the lesson window

    • Apply it: A device’s settings show a prefix of /22. What is its subnet mask?
    • Apply it: A colleague configures the mask 255.255.0.255. What is wrong with it?

  9. 8. 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 131.107.5.15 question links back to the CIDR step.
    • 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 2 min

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

    Ask

    • Without looking: convert /26 to a mask.

    Responses to expect

    • 255.255.255.192.
    • 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

    • The boundary decides local or remote, and how many hosts fit.
    • Each extra host bit doubles the room.
    • Classes were implied and wasteful; prefixes are configured.
    • A mask is ones then zeros, never mixed.
    • Count the ones to get the prefix.

Consolidation

  • The boundary decides local or remote, and how many hosts fit.
  • Each extra host bit doubles the room.
  • Classes were implied and wasteful; prefixes are configured.
  • A mask is ones then zeros, never mixed.
  • Count the ones to get the prefix.