3.1 Ethernet Overview

Ethernet is a common technology used to connect devices on wired local networks. Computers, printers, switches, routers, access points, and many other devices can use Ethernet connections.

Think of Ethernet as a set of communication rules. The rules help devices know how data should be packaged, addressed, sent, received, and checked.

Very simple example

Computer A ---- Switch ---- Computer B

Computer A wants to send information to Computer B. It places the information inside an Ethernet frame. The switch reads addressing information in the frame and forwards it toward Computer B.

Real-world example

If your desktop computer is connected to a home router or network switch with a network cable, Ethernet may be carrying the data between those devices.

3.2 Networking Standards

A networking standard is an agreed technical specification that helps equipment from different manufacturers work together.

Without common standards, one company's network adapter might use rules that another company's switch does not understand.

Example

Laptop from Manufacturer A
          |
          | standard Ethernet connection
          |
Switch from Manufacturer B

The two devices can communicate because they follow compatible technical rules.

Why standards matter

• Compatibility between devices
• Predictable cable and link behavior
• Common frame formats
• Common speed and duplex capabilities

3.3 Ethernet Frames

An Ethernet frame is a structured unit of data sent across an Ethernet network. When a device sends network information, Ethernet adds fields that help deliver and check the data.

Simplified frame idea

+----------------------+----------------------+-----------+---------+-------+
| Destination Address  | Source Address       | Type      | Data    | Check |
+----------------------+----------------------+-----------+---------+-------+

The real frame contains defined fields, but this simplified picture is enough for a beginner to understand the job of the frame.

Example

A computer sends a web request. The request becomes part of network data, and Ethernet carries that data inside a frame across the local Ethernet link.

3.4 Source MAC Address

The source MAC address identifies the network interface that sent the Ethernet frame onto the current local Ethernet segment.

Example

Computer A ---- Switch ---- Computer B
    |
    +-- Source MAC = Computer A's local interface address

When Computer A sends the frame, its MAC address normally appears in the source-address field.

Think of it as the sender address on the local Ethernet frame.

3.5 Destination MAC Address

The destination MAC address tells Ethernet devices which local network interface should receive the frame, or what type of local delivery is intended.

Example

Computer A ---- Switch ---- Computer B
                                |
                                +-- Destination MAC = Computer B

A switch can use the destination MAC address to decide where to forward the frame.

Important beginner idea

The source MAC answers "Who sent it?" and the destination MAC answers "Who should receive it locally?"

3.6 EtherType

EtherType is a field used in Ethernet framing to identify what kind of higher-level protocol or payload follows.

It helps the receiving device understand how to interpret the data carried inside the frame.

Simple analogy

Imagine receiving a box with a label that says what kind of material is inside. EtherType acts like that label for the data carried by the Ethernet frame.

Ethernet Frame
   |
   +-- Addresses
   +-- Type information
   +-- Payload data
   +-- Error-checking information

3.7 Frame Check Sequence

The Frame Check Sequence, often shortened to FCS, helps a receiving Ethernet device detect whether a frame was damaged while traveling across the link.

The sender calculates checking information and places it in the frame. The receiver performs its own calculation and compares the result.

Example

Sender ---- frame ----> Receiver
              |
              +-- FCS used to help detect transmission errors

If the check shows that the frame is corrupted, the damaged frame is normally discarded.

3.8 MAC Addresses

A MAC address is a link-layer address associated with a network interface. Ethernet uses MAC addresses for local frame delivery.

MAC addresses are commonly written as groups of hexadecimal characters.

Example format

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

This example is only a demonstration format. It is not intended to identify a real device.

MAC address versus IP address

An easy beginner distinction is:

• MAC address: used for local link-layer delivery.
• IP address: used for network-layer addressing and routing between networks.

3.9 Unicast

Unicast means communication intended for one specific destination.

Example

Computer A ----------> Computer B
        one sender        one intended receiver

If your computer sends an Ethernet frame specifically to another local device, that is a unicast-style delivery.

Simple analogy

Unicast is like sending a private letter to one person.

3.10 Broadcast

Broadcast means a frame is intended for all devices in the local broadcast domain.

Example idea

              +--> Device A
Sender ------ +--> Device B
              +--> Device C
              +--> Device D

Every device in the same broadcast domain can receive the broadcast frame, although each device decides how to process it.

Simple analogy

Broadcast is like making an announcement over a room's loudspeaker so everyone in the room can hear it.

3.11 Multicast

Multicast means sending traffic to a selected group of interested receivers rather than to only one receiver or to everybody.

Example idea

             +--> Group Member A
Sender ----- +--> Group Member B
             +--> Group Member C

Other devices are not intended recipients.

Multicast can be useful when the same information needs to reach multiple participating devices efficiently.

Simple comparison

• Unicast = one destination
• Broadcast = all local devices
• Multicast = selected group

3.12 Collision Domains

A collision domain is an area of a network where simultaneous transmissions on a shared medium could interfere with each other in older shared Ethernet designs.

Modern switched full-duplex Ethernet greatly reduces the practical collision problems associated with older shared-media networks.

Older shared-medium idea

PC A ---- Shared Medium ---- PC B
              |
             PC C

If two devices tried to transmit at the same time on a shared half-duplex medium, a collision could occur.

Modern switch idea

PC A ---- Switch ---- PC B
           |
          PC C

Each switch port creates a separate Ethernet segment, helping isolate traffic and avoid the old shared-medium collision behavior.

3.13 Broadcast Domains

A broadcast domain is the group of devices that can receive one another's local Layer 2 broadcasts.

A normal Layer 2 switch forwards broadcasts within the same broadcast domain. A router or Layer 3 boundary normally separates broadcast domains.

Example

PC A -- Switch -- PC B
          |
         PC C
          |
        Router
          |
       Other Network

A broadcast from PC A can reach devices in its local broadcast domain but is not normally forwarded by the router into the other network.

3.14 Full Duplex

Full duplex allows both ends of a link to send and receive data at the same time.

Example

Computer  <==========>  Switch
          send + receive
          at the same time

Modern switched Ethernet links commonly operate in full-duplex mode when both ends support and agree on it.

Simple analogy

Full duplex is like a normal phone conversation where both people can speak and listen at the same time.

3.15 Half Duplex

Half duplex allows communication in both directions, but not at the same time.

Example idea

Step 1: Device A -----> Device B
Step 2: Device A <----- Device B

One side transmits, then the other side can transmit.

Simple analogy

Half duplex is similar to using a basic push-to-talk radio: one person talks while the other listens, then they switch.

Half-duplex Ethernet is mainly associated with older or specialized shared-medium situations.

3.16 Auto-Negotiation

Auto-negotiation is a process that allows connected Ethernet devices to automatically determine compatible link settings.

Depending on the equipment and technology, devices can negotiate capabilities such as speed and duplex.

Example

Computer Network Adapter
        |
        |  compare supported capabilities
        |
      Switch Port
        |
        +--> choose compatible link settings

This makes Ethernet setup easier because users often do not need to configure every link parameter manually.

Possible problem

If one end is manually forced to settings that do not match the other end, performance or connectivity problems can occur.

3.17 Ethernet Speeds

Ethernet has developed over time to support many different data rates. The actual speed available depends on the network interfaces, switches, cabling or fiber, transceivers, distance, and supported standards.

Common speed families

• 10 megabits per second
• 100 megabits per second
• 1 gigabit per second
• 10 gigabits per second
• Higher multi-gigabit and high-speed Ethernet rates

Example

If a computer supports 1 Gbps but it connects to equipment limited to 100 Mbps, the link cannot operate faster than the mutually supported capability of that connection.

3.18 10 Mbps Ethernet

10 Mbps Ethernet represents an early Ethernet speed of ten megabits per second.

It is very slow compared with modern Ethernet, but it is important historically because it helps show how Ethernet evolved.

Speed comparison

10 Mbps
100 Mbps   = 10 times the nominal data rate
1 Gbps     = 100 times the nominal data rate of 10 Mbps

Modern networks usually use much faster links, but older concepts can still appear in networking education and legacy environments.

3.19 Fast Ethernet

Fast Ethernet commonly refers to the 100 Mbps Ethernet generation.

It increased Ethernet's data rate from the earlier 10 Mbps generation to 100 Mbps while keeping Ethernet's general frame-based communication model.

Example

Older link:   10 Mbps
Faster link: 100 Mbps

A 100 Mbps network interface can theoretically move data at a higher link rate than a 10 Mbps interface, although real application throughput is affected by many other factors.

3.20 Gigabit Ethernet

Gigabit Ethernet commonly refers to Ethernet operating at 1 gigabit per second, which equals 1,000 megabits per second in standard networking rate notation.

Example

Computer -- 1 Gbps link -- Switch -- 1 Gbps link -- Server

Gigabit Ethernet became very common for desktop, home, school, and business LAN connections.

Important beginner point

A gigabit-capable device does not guarantee gigabit application performance. Storage speed, server performance, protocol overhead, other network traffic, and the Internet connection can all affect actual results.

3.21 10 Gigabit Ethernet and Faster

10 Gigabit Ethernet provides a nominal Ethernet link rate of 10 gigabits per second. Faster Ethernet technologies are also used in data centers, service-provider networks, high-performance computing, storage networks, and enterprise backbones.

Example

User PCs ---- Access Switch ---- High-Speed Uplink ---- Core / Servers

The user devices might use lower-speed access links while the switch uses a much faster uplink to carry the combined traffic of many users.

Why faster Ethernet is used

• Large numbers of users
• Server-to-server traffic
• Large file transfers
• Virtualization
• Storage traffic
• Network backbone connections

Chapter 3 Beginner Summary