7.1 What Is Ethernet?

Ethernet is the most common technology used for wired local area networks.

Think:

Ethernet = wired LAN communication standard

A simple Ethernet network might look like this:

PC1 ----\
PC2 ----- Switch ---- Router
PC3 ----/

The PCs connect to the switch using Ethernet cables.

Ethernet defines things such as:

• Frame format
• MAC addressing
• Cable types
• Signaling
• Speeds
• Duplex operation

7.2 IEEE 802.3

Ethernet standards are defined mainly by:

IEEE 802.3

IEEE stands for:

Institute of Electrical and Electronics Engineers

You do not need to memorize every IEEE standard number, but remember:

802.3  = Ethernet

802.11 = Wi-Fi

That distinction appears often in networking exams.

7.3 Ethernet Frames

Ethernet sends data using frames.

A simplified Ethernet frame looks like:

+----------------------+
| Destination MAC      |
+----------------------+
| Source MAC           |
+----------------------+
| Type/Length          |
+----------------------+
| Data                 |
+----------------------+
| FCS / Error Check    |
+----------------------+

The important beginner fields are:

• Destination MAC address
• Source MAC address
• Payload/data
• Error-checking information

Example

Suppose Computer A wants to send data to Computer B.

Computer A
MAC: AA-AA-AA-AA-AA-AA

Computer B
MAC: BB-BB-BB-BB-BB-BB

The Ethernet frame may contain:

Source MAC:
AA-AA-AA-AA-AA-AA

Destination MAC:
BB-BB-BB-BB-BB-BB

The switch reads the destination MAC and forwards the frame.

7.4 MAC Addresses

A MAC address is a Layer 2 address associated with a network interface.

MAC stands for:

Media Access Control

A MAC address is usually 48 bits long.

Example

00:1A:2B:3C:4D:5E

It may also be written:

00-1A-2B-3C-4D-5E

Remember:

MAC address
= Layer 2

IP address
= Layer 3

7.5 Organizationally Unique Identifier — OUI

The first portion of a traditional MAC address identifies the manufacturer.

This part is called the:

OUI — Organizationally Unique Identifier

Example

00:1A:2B:3C:4D:5E

00:1A:2B
   ↓
Manufacturer identifier

The remaining portion identifies the individual interface.

Why is OUI useful?

It can help identify which company manufactured a network interface.

For example, network tools may recognize a MAC address as belonging to:

• Cisco
• Intel
• Apple
• Dell
• HP

7.6 Unicast

Unicast means communication from one sender to one specific receiver.

Think:

One-to-one

Example

Computer A --------> Computer B

Computer A sends a frame specifically to Computer B's MAC address.

Most normal network communication is unicast.

Example

Your laptop connects to a web server.

Laptop → Server

That is primarily unicast communication.

7.7 Broadcast

A broadcast is sent from one device to all devices in the local broadcast domain.

Think:

One-to-everyone locally

An Ethernet broadcast destination MAC is:

FF:FF:FF:FF:FF:FF

Example

             PC1
              ↑
              |
PC2 <----- Switch -----> PC3
              |
              ↓
             PC4

If a broadcast enters the switch, the switch forwards it out appropriate ports within that broadcast domain.

Example use

ARP commonly uses broadcasts in IPv4 LANs.

A device may effectively ask:

"Who has IP address 192.168.1.20?"

All local devices receive the broadcast, but only the device with that IP should answer.

7.8 Multicast

Multicast means communication from one sender to a selected group of receivers.

Think:

One-to-many, but not everyone

Example

              PC1 ✓
                ↑
                |
Server ---- Network ---- PC2 ✓
                |
                ↓
              PC3 ✗

PC1 and PC2 are members of the multicast group.

PC3 is not.

Multicast can be useful for:

• Streaming
• Routing protocols
• Video distribution
• Group communication

7.9 Collision Domains

A collision domain is a network area where Ethernet transmissions could collide.

This was much more important in older shared Ethernet networks.

Imagine two devices trying to transmit at exactly the same time on the same shared medium.

PC A ----\
          Shared medium
PC B ----/

The signals could interfere.

That event is called a:

Collision

Modern switches

A switch separates collision domains.

Each switch port is effectively its own collision domain.

Example

PC1 ---- Port 1

PC2 ---- Port 2

PC3 ---- Port 3

Each switch port provides a separate collision domain.

7.10 Broadcast Domains

A broadcast domain is the group of devices that receive a Layer 2 broadcast.

A basic Layer 2 switch does not normally stop broadcasts.

Example

PC1
 |
Switch
 |
PC2
 |
PC3

If PC1 sends a broadcast, PC2 and PC3 may receive it.

A router separates broadcast domains.

Example

LAN A
  |
Router
  |
LAN B

Broadcast traffic from LAN A normally does not cross the router into LAN B.

Switch
separates collision domains

Router
separates broadcast domains

7.11 CSMA/CD

CSMA/CD stands for:

Carrier Sense Multiple Access with Collision Detection

It was used in older shared, half-duplex Ethernet.

Carrier Sense

A device listens before transmitting.

It asks:

"Is anyone else transmitting?"

Multiple Access

Multiple devices share the same medium.

Collision Detection

If a collision happens, the device detects it.

Then devices wait and retry.

Simplified

Listen
  ↓
Medium free?
  ↓
Transmit
  ↓
Collision?
  ↓
Wait
  ↓
Retry

Modern Ethernet

Modern switched full-duplex Ethernet does not normally experience collisions, so CSMA/CD is mostly a historical concept now.

But it remains important for Networking understanding.

7.12 Half Duplex

Half duplex means communication can travel in both directions, but not at the same time.

Think of a walkie-talkie.

Person A:

"Hello"

Then stops.

Person B:

"Hello back"

Only one person talks at a time.

Ethernet example

A -------- B

At one moment:

A → B

Later:

A ← B

but not both simultaneously.

Half-duplex disadvantages

• Collisions can occur
• Lower effective performance
• More waiting

Modern switched Ethernet usually uses full duplex.

7.13 Full Duplex

Full duplex allows communication in both directions at the same time.

Think of a telephone conversation.

A ↔ B

Both sides can send and receive simultaneously.

Ethernet example

PC ↔ Switch

The PC can transmit while receiving.

Benefits

• No collisions on the point-to-point switched link
• Better performance
• More efficient communication

Half duplex
= one direction at a time

Full duplex
= both directions simultaneously

7.14 Auto-Negotiation

Auto-negotiation allows Ethernet devices to automatically agree on connection settings.

These may include:

• Speed
• Duplex

Example

PC supports:

• 100 Mbps
• 1 Gbps

Switch supports:

• 100 Mbps
• 1 Gbps

Result:

1 Gbps full duplex

The devices usually choose the best mutually supported settings.

Why is this helpful?

Without auto-negotiation, technicians would need to manually configure speed and duplex more often.

7.15 Duplex Mismatch

A duplex mismatch happens when two sides of an Ethernet link use different duplex settings.

Example

PC:
Full duplex

Switch:
Half duplex

This can cause:

• Slow performance
• Errors
• Retransmissions
• Collisions on the half-duplex side
• Strange intermittent connectivity

7.16 Ethernet Standards

Ethernet exists in many speed and media standards.

Older and modern examples include:

• 10 Mbps Ethernet
• 100 Mbps Fast Ethernet
• 1 Gbps Gigabit Ethernet
• 10 Gbps Ethernet
• 40 Gbps Ethernet
• 100 Gbps Ethernet
• Higher-speed data-center Ethernet

You will study cabling requirements in later chapters.

7.17 Fast Ethernet

Fast Ethernet generally refers to:

100 Mbps Ethernet

One common standard is:

100BASE-TX

Breaking the name down:

100
= 100 Mbps

BASE
= baseband

TX
= twisted-pair implementation

Fast Ethernet was much faster than older 10 Mbps Ethernet, but 1 Gbps Ethernet is now very common.

7.18 Gigabit Ethernet

Gigabit Ethernet usually means:

1 Gbps Ethernet

1 Gbps equals:

1000 Mbps

A common copper Ethernet standard is:

1000BASE-T

It commonly runs over twisted-pair Ethernet cabling.

Example

PC ===== 1 Gbps ===== Switch

Gigabit Ethernet is extremely common in:

• Homes
• Offices
• Schools
• Small businesses

7.19 10 Gigabit Ethernet

10 Gigabit Ethernet provides:

10 Gbps

That is:

10,000 Mbps

10 Gigabit Ethernet is often used for:

• Server connections
• Switch uplinks
• Data centers
• High-performance workstations
• Network backbones

Examples include:

• 10GBASE-T
• Various fiber-based standards

7.20 Higher-Speed Ethernet

Large data centers and enterprise networks may use:

• 25 Gbps
• 40 Gbps
• 50 Gbps
• 100 Gbps
• 200 Gbps
• 400 Gbps and beyond

You do not need to memorize every implementation for basic Networking understanding.

How a Switch Learns MAC Addresses

This is an important Ethernet concept.

Suppose we have:

PC A ----\
          Switch
PC B ----/

PC A sends a frame.

The switch looks at:

Source MAC address

Suppose:

PC A MAC:

AA:AA:AA:AA:AA:AA

The switch learns:

MAC AA:AA:AA:AA:AA:AA

is reachable through Port 1

Its MAC table might look like:

Chapter 7 Practice Questions