Chapter 4 — Session 3: MAC Addresses & Ethernet Addressing

Now we move from binary/hexadecimal into MAC addressing. This is a major Network+ area because Ethernet uses MAC addresses for Layer 2 local delivery. Your Chapter 4 material introduces MAC addresses as 48-bit / 6-byte hexadecimal addresses.

📋 Session 3 — Topic Checklist

Use this to track what you know:

  • ⬜ MAC Address

  • ⬜ 48-bit / 6-byte MAC Length

  • ⬜ Hexadecimal MAC Format

  • ⬜ OUI — Organizationally Unique Identifier

  • ⬜ Vendor Portion vs Device Portion

  • ⬜ Unicast MAC Address

  • ⬜ Multicast MAC Address

  • ⬜ Broadcast MAC Address

  • ⬜ I/G Bit — Individual/Group

  • ⬜ U/L Bit — Universal/Local

  • ⬜ Locally Administered MAC Address

  • ⬜ MAC Address vs IP Address

📝 Notebook Notes — Quick Review

🪪 MAC Address

MAC = Media Access Control

A MAC address identifies a network interface at:

⭐ OSI Layer 2 — Data Link

Ethernet uses MAC addresses to deliver frames on the local network.

Network+ Must Memorize

A MAC address is:

48 bits

which equals:

6 bytes

and is normally displayed as:

12 hexadecimal characters

Example:

00:0B:DB:99:D3:5E

Your chapter identifies MAC addresses as 48-bit addresses represented in hexadecimal.

🧠 Memory

MAC = 48 → 6 → 12

48 bits → 6 bytes → 12 hex digits

🧮 Why 12 Hex Characters?

Remember Session 2:

1 hex digit = 4 bits

Therefore:

12 hex digits × 4 bits
        =
      48 bits

Or:

00 : 0B : DB : 99 : D3 : 5E
↓    ↓    ↓    ↓    ↓    ↓
1    2    3    4    5    6 bytes

⭐ There are 6 groups of 2 hexadecimal digits.

🏭 OUI

OUI = Organizationally Unique Identifier

The OUI identifies the manufacturer/vendor associated with the MAC address.

Think:

OUI = WHO MADE IT? 🏭

Traditionally, the first half of a globally assigned MAC address contains the vendor OUI.

Example:

00:0B:DB : 99:D3:5E
────────   ────────
 Vendor      Device
  OUI        portion

🧠 Memory

OUI = Organization = Manufacturer

⭐ MAC Address Structure

A 48-bit MAC can be thought of as:

        48-bit MAC
             │
     ┌───────┴───────┐
     ↓               ↓
 First 24 bits    Last 24 bits
     ↓               ↓
    OUI          Device-specific
     ↓               ↓
 Manufacturer       Interface

For exam purposes:

First half = vendor

Second half = device/interface-specific

👤 Unicast MAC

Unicast = ONE destination

PC A ───────────▶ PC B

One sender sends a frame to one specific interface.

🧠 Memory:

UNI = ONE

Like:

Unicycle = one wheel 🚲

Unicast = one destination

👥 Multicast MAC

Multicast = GROUP

A multicast frame is intended for a specific group of devices.

               ┌──▶ PC A
Sender ────────┼──▶ PC B
               └──▶ PC C

             GROUP

Not necessarily everyone.

🧠 Memory

MULTI = Multiple members 👥

📢 Broadcast MAC

Broadcast means:

EVERYONE on the local broadcast domain

The Ethernet broadcast MAC address is:

⭐ FF:FF:FF:FF:FF:FF

🔥 MEMORIZE THIS COLD

Sender
   │
   ├────▶ PC 1
   ├────▶ PC 2
   ├────▶ PC 3
   └────▶ PC 4

🧠 Memory:

ALL F's = ALL devices 📢

⭐ Three Destination Types

Type

Destination

Memory

Unicast

One device

👤 UNI = ONE

Multicast

Selected group

👥 MULTI = GROUP

Broadcast

Everyone locally

📢 ALL

This distinction is extremely important for Network+.

🧠 I/G Bit

I/G = Individual / Group

This bit helps indicate whether the MAC address represents:

Individual

➡ Unicast

or

Group

➡ Multicast

🧠 Memory

I = Individual 👤

G = Group 👥

Don't make this harder than it needs to be.

🏠 U/L Bit

U/L = Universal / Local

This indicates whether an address is:

Universal

Globally/manufacturer assigned

or

Local

Locally administered

🧠 Memory

U = Universal 🌎

L = Local 🏠

🔧 Locally Administered MAC

Normally, a device may use its manufacturer-assigned MAC.

But software/administrators can sometimes assign or use a different MAC address.

That's a:

Locally administered MAC address

Think:

Manufacturer MAC
      ↓

🌎 Universal Administrator/software changes it ↓ 🏠 Local

💻 Knowt Notes — Deep Understanding

🆚 MAC Address vs IP Address

This distinction is VERY important.

MAC Address

Think:

WHO is the interface locally?

Layer:

⭐ Layer 2

Used for:

Local Ethernet frame delivery

IP Address

Think:

WHERE is the device logically located?

Layer:

⭐ Layer 3

Used for:

Routing between networks

🧠 Easy Analogy

Imagine sending a package.

IP Address = Street Address 🏠

It gets you to the correct location/network.

MAC Address = Person's Name 🪪

Once you're there, it helps identify the local destination interface.

Simplified:

Internet
   │
   │ IP
   ▼
Your LAN
   │
   │ MAC
   ▼
Your Device

🚨 Important Concept

Suppose:

PC A
192.168.1.10

wants to communicate with

PC B
192.168.1.20

Because PC B is local, PC A ultimately needs PC B's:

⭐ MAC address

to build the Ethernet frame.

That's why the earlier part of your chapter showed devices learning the information necessary for local Ethernet communication.

📦 Think Ahead to Ethernet Frames

We're going deeper into this next, but remember:

Ethernet Frame
┌──────────────────────────────┐
│ Destination MAC             │
│ Source MAC                  │
│ Data                        │
│ FCS                         │
└──────────────────────────────┘

MAC addresses are placed inside the Ethernet frame.

That is why MAC addressing belongs to:

⭐ Layer 2

⚠ EXAM TRAPS

🚨 Trap #1

MAC address = 32 bits

❌ Wrong.

IPv4 addresses are 32 bits.

MAC:

✅ 48 bits

🚨 Trap #2

MAC address = Layer 3

❌ Wrong.

MAC:

✅ Layer 2

IP:

✅ Layer 3

🚨 Trap #3

FF:FF:FF:FF:FF:FF is multicast.

❌ Wrong.

It is:

⭐ BROADCAST

🚨 Trap #4

Unicast means everyone.

❌ Wrong.

UNI = ONE

🚨 Trap #5

Multicast means everyone.

❌ Wrong.

Multicast = selected group

Broadcast = everyone

🎴 Multiple-Choice Flashcards

⭐ 1. EXAM GOLD

How long is a standard MAC address?

A. 32 bits
B. 48 bits
C. 64 bits
D. 128 bits

✅ Answer: B — 48 bits

🧠 Memory: MAC = 48.

⭐ 2.

How many bytes are in a 48-bit MAC address?

A. 4
B. 6
C. 8
D. 12

✅ Answer: B — 6 bytes

🧠 Memory: 48 ÷ 8 = 6.

⭐ 3.

How many hexadecimal characters normally represent a 48-bit MAC address?

A. 6
B. 8
C. 12
D. 16

✅ Answer: C — 12

🧠 Memory: MAC = 48 → 6 bytes → 12 hex.

⭐ 4. EXAM GOLD

At which OSI layer are MAC addresses primarily used?

A. Layer 1
B. Layer 2
C. Layer 3
D. Layer 4

✅ Answer: B — Layer 2

🧠 Memory: MAC = Layer 2.

⭐ 5. EXAM GOLD

What does OUI identify?

A. IP subnet
B. Manufacturer/vendor
C. TCP port
D. Default gateway

✅ Answer: B — Manufacturer/vendor

🧠 Memory: OUI = Organization. 🏭

6.

Traditionally, which portion of a globally assigned 48-bit MAC address contains the OUI?

A. First 24 bits
B. Last 8 bits
C. Last 24 bits
D. All 48 bits

✅ Answer: A — First 24 bits

🧠 Memory: Vendor first → device second.

⭐ 7.

Which transmission is intended for ONE specific destination?

A. Broadcast
B. Multicast
C. Unicast
D. Anycast

✅ Answer: C — Unicast

🧠 Memory: UNI = ONE. 👤

⭐ 8.

Which transmission is intended for a selected group?

A. Unicast
B. Multicast
C. Broadcast
D. Half-duplex

✅ Answer: B — Multicast

🧠 Memory: MULTI = group. 👥

⭐ 9. EXAM GOLD

What is the Ethernet broadcast MAC address?

A. 00:00:00:00:00:00
B. FF:00:FF:00:FF:00
C. FF:FF:FF:FF:FF:FF
D. 255:255:255:255

✅ Answer: C — FF:FF:FF:FF:FF:FF

🧠 Memory: ALL F's = ALL devices. 📢

10.

What does the I/G bit indicate?

A. Internet/Gateway
B. Individual/Group
C. Internal/Global
D. Interface/Gateway

✅ Answer: B — Individual/Group

🧠 Memory: I = Individual, G = Group.

⭐ 11.

What does the U/L bit indicate?

A. Upload/Link
B. Universal/Local
C. Unicast/Loopback
D. User/LAN

✅ Answer: B — Universal/Local

🧠 Memory: U = Universal 🌎 / L = Local 🏠

⭐ 12. EXAM GOLD

Which pairing is correct?

A. MAC → Layer 3; IP → Layer 2
B. MAC → Layer 2; IP → Layer 3
C. MAC → Layer 1; IP → Layer 4
D. MAC → Layer 4; IP → Layer 1

✅ Answer: B

🧠 Memory: MAC 2, IP 3.

13.

A frame needs to be delivered to every device in the local broadcast domain. Which destination MAC should be used?

A. 00:00:00:00:00:00
B. FF:FF:FF:FF:FF:FF
C. The default gateway's MAC only
D. The sender's MAC

✅ Answer: B

🧠 Memory: All F = all devices.

⭐ 14. Scenario

A computer is sending an Ethernet frame directly to one other computer on its LAN. What type of destination address is normally being used?

A. Broadcast
B. Multicast
C. Unicast
D. Loopback

✅ Answer: C — Unicast

🧠 Memory: One PC → one PC = UNIcast.

🚨 15. EXAM TRAP

Which statement is correct?

A. Broadcast targets one host
B. Multicast always targets every LAN device
C. Unicast targets a specific destination
D. MAC addresses operate at Layer 3

✅ Answer: C

🧠 Memory:

UNI = One

MULTI = Group

BROAD = Everyone

🧠 Memory Board

🪪 MAC = Layer 2 🔢 MAC = 48 bits 📦 MAC = 6 bytes 🔤 MAC = 12 hex characters 🏭 OUI = Manufacturer 👤 UNICAST = ONE 👥 MULTICAST = GROUP 📢 BROADCAST = EVERYONE 📢 Broadcast MAC: FF:FF:FF:FF:FF:FF I/G I = Individual G = Group U/L U = Universal L = Local MAC = WHO locally IP = WHERE logically

🏆 Network+ Summary

The big picture is:

Ethernet uses MAC addresses at Layer 2.

A MAC address is:

48 bits → 6 bytes → 12 hexadecimal characters

The OUI helps identify the manufacturer, while Ethernet destination addressing can be unicast, multicast, or broadcast.

Your Chapter 4 material introduces the 48-bit hexadecimal MAC-address structure here.

🎯 Network+ Exam Priorities

🔥 Know these instantly:

MAC → Layer 2

MAC → 48 bits

MAC → 6 bytes

MAC → 12 hex characters

OUI → Vendor

Unicast → ONE 👤

Multicast → GROUP 👥

Broadcast → EVERYONE 📢

Broadcast MAC → FF:FF:FF:FF:FF:FF

I/G → Individual/Group

U/L → Universal/Local

MAC = Layer 2 / IP = Layer 3

➡ Next Session

Chapter 4 — Session 4: Ethernet Frames

Your checklist will include:

⬜ Ethernet Frame
⬜ Preamble
⬜ SFD
⬜ Destination MAC
⬜ Source MAC
⬜ EtherType/Length
⬜ Payload/Data
⬜ FCS
⬜ CRC
⬜ Minimum Frame Size
⬜ Maximum Frame Size
⬜ MTU
⬜ Jumbo Frames