CCNA学习指南v2.71：超清英文版入门教程

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"《CCNA学习指南超清版》是一份专门为希望获得Cisco Certified Network Associate (CCNA)认证的学习者编写的参考资料。由Aaron Balchunas编著，该指南在2014年发布，版本号为v2.71。这份文档的主要目标是为学习者提供一个学习框架，明确他们需要掌握的核心概念，而非全面的考试策略或问题答案集。它鼓励读者通过自我学习和深入理解来逐步提升技能，而不是依赖于纯粹的记忆技巧。版权方面，所有原始材料均受Aaron Balchunas保护，除非另有注明，否则未经其许可不得复制或改动。其他部分内容可能来自其各自所有者的版权。学习者可以在不改变或销售文档的前提下，自由地复制和使用这份材料。更新内容可以在<a href="http://www.routeralley.com">http://www.routeralley.com</a>网站上找到。文档前言强调了其非官方性质，旨在提供一个学习路径，并不是完整的考试准备工具，也不是标准的题库。作者明确表示，尽管内容可以免费分发，但任何对文档内容的修改或商业化使用都需要得到他的书面同意。这份指南的目的是促进知识共享和学习过程，而非单纯为了通过考试而应试。在《CCNA学习指南》中，读者可以期待到网络基础知识、网络设备操作、路由与交换技术、网络安全、配置和故障排除等核心课程的概述和讲解。每章内容详实，旨在帮助读者建立起扎实的理论基础和实践经验，从而为CCNA认证考试做好充分准备。通过耐心学习和深入实践，相信学习者会在这个过程中有所收获并逐渐掌握CCNA所需的技能。"

CCNA Study Guide v2.71 – Aaron Balchunas

* * *

Aaron Balchunas (aaron@routeralley.com),

This material may be copied and used freely, but may not be altered or sold without the expressed written

consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.

Encapsulation Illustrated

The following illustrates how basic encapsulation occurs with the TCP/IP

stack, which typically performs encapsulation only at the lower layers:

During encapsulation on the sending host:

• Data from the user application is handed off to the Transport layer.

• The Transport layer adds a header containing protocol-specific

information, and then hands the segment to the Network layer.

• The Network layer adds a header containing source and destination

logical addressing, and then hands the packet to the Data-Link layer.

• The Data-Link layer adds a header containing source and destination

physical addressing and other hardware-specific information.

• The Data-Link frame is then handed off to the Physical layer to be

transmitted on the network medium as bits.

During decapsulation on the receiving host, the reverse occurs:

• The frame is received from the physical medium.

• The Data-Link layer processes its header, strips it off, and then hands

it off to the Network layer.

• The Network layer processes its header, strips it off, and then hands it

off to the Transport layer.

• The Transport layer processes its header, strips it off, and then hands

the data to the user application.

CCNA Study Guide v2.71 – Aaron Balchunas

* * *

Aaron Balchunas (aaron@routeralley.com),

This material may be copied and used freely, but may not be altered or sold without the expressed written

consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.

OSI Reference Model Example

A web browser serves as a good practical illustration of the OSI model and

the TCP/IP protocol suite:

• Τhe web browser serves as the user interface for accessing a website. The

browser itself does not function at the Application layer. Instead, the

web browser invokes the Hyper Text Transfer Protocol (HTTP) to

interface with the remote web server, which is why http:// precedes every

web address.

• The Internet can provide data in a wide variety of formats, a function of

the Presentation layer. Common formats on the Internet include HTML,

XML, PHP, GIF, and JPEG. Any encryption or compression mechanisms

used on a website are also considered a Presentation layer function.

• The Session layer is responsible for establishing, maintaining, and

terminating the session between devices, and determining whether the

communication is half-duplex or full-duplex. However, the TCP/IP stack

generally does not include session-layer protocols, and is reliant on

lower-layer protocols to perform these functions.

• HTTP utilizes the TCP Transport layer protocol to ensure the reliable

delivery of data. TCP establishes and maintains a connection from the

client to the web server, and packages the higher-layer data into

segments. A sequence number is assigned to each segment so that data

can be reassembled upon arrival.

• The best path to route the data between the client and the web server is

determined by IP, a Network layer protocol. IP is also responsible for

the assigned logical addresses on the client and server, and for

encapsulating segments into packets.

• Data cannot be sent directly to a logical address. As packets travel from

network to network, IP addresses are translated to hardware addresses,

which are a function of the Data-Link layer. The packets are

encapsulated into frames to be placed onto the physical medium.

• The data is finally transferred onto the network medium at the Physical

layer, in the form of raw bits. Signaling and encoding mechanisms are

defined at this layer, as is the hardware that forms the physical

connection between the client and the web server.

CCNA Study Guide v2.71 – Aaron Balchunas

* * *

Aaron Balchunas (aaron@routeralley.com),

This material may be copied and used freely, but may not be altered or sold without the expressed written

consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.

Section 3

- Ethernet Technologies -

What is Ethernet?

Ethernet is a family of technologies that provides data-link and physical

specifications for controlling access to a shared network medium. It has

emerged as the dominant technology used in LAN networking.

Ethernet was originally developed by Xerox in the 1970s, and operated at

2.94Mbps. The technology was standardized as Ethernet Version 1 by a

consortium of three companies - DEC, Intel, and Xerox, collectively referred

to as DIX - and further refined as Ethernet II in 1982.

In the mid 1980s, the Institute of Electrical and Electronic Engineers

(IEEE) published a formal standard for Ethernet, defined as the IEEE 802.3

standard. The original 802.3 Ethernet operated at 10Mbps, and successfully

supplanted competing LAN technologies, such as Token Ring.

Ethernet has several benefits over other LAN technologies:

• Simple to install and manage

• Inexpensive

• Flexible and scalable

• Easy to interoperate between vendors

(References: http://docwiki.cisco.com/wiki/Ethernet_Technologies; http://www.techfest.com/networking/lan/ethernet1.htm)

Ethernet Cabling Types

Ethernet can be deployed over three types of cabling:

• Coaxial cabling – almost entirely deprecated in Ethernet networking

• Twisted-pair cabling

• Fiber optic cabling

Coaxial cable, often abbreviated as coax, consists of a single wire

surrounded by insulation, a metallic shield, and a plastic sheath. The shield

helps protect against electromagnetic interference (EMI), which can cause

attenuation, a reduction of the strength and quality of a signal. EMI can be

generated by a variety of sources, such as florescent light ballasts,

microwaves, cell phones, and radio transmitters.

Coax is commonly used to deploy cable television to homes and businesses.

CCNA Study Guide v2.71 – Aaron Balchunas

* * *

Aaron Balchunas (aaron@routeralley.com),

This material may be copied and used freely, but may not be altered or sold without the expressed written

consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.

Ethernet Cabling Types (continued)

Two types of coax were used historically in Ethernet networks:

• Thinnet

• Thicknet

Thicknet has a wider diameter and more shielding, which supports greater

distances. However, it is less flexible than the smaller thinnet, and thus more

difficult to work with. A vampire tap is used to physically connect devices

to thicknet, while a BNC connector is used for thinnet.

Twisted-pair cable consists of two or four pairs of copper wires in a plastic

sheath. Wires in a pair twist around each other to reduce crosstalk, a form of

EMI that occurs when the signal from one wire bleeds or interferes with a

signal on another wire. Twisted-pair is the most common Ethernet cable.

Twisted-pair cabling can be either shielded or unshielded. Shielded twisted-

pair is more resistant to external EMI; however, all forms of twisted-pair

suffer from greater signal attenuation than coax cable.

There are several categories of twisted-pair cable, identified by the number

of twists per inch of the copper pairs:

• Category 3 or Cat3 - three twists per inch.

• Cat5 - five twists per inch.

• Cat5e - five twists per inch; pairs are also twisted around each other.

• Cat6 – six twists per inch, with improved insulation.

An RJ45 connector is used to connect a device to a twisted-pair cable. The

layout of the wires in the connector dictates the function of the cable.

While coax and twisted-pair cabling carry electronic signals, fiber optics

uses light to transmit a signal. Ethernet supports two fiber specifications:

• Singlemode fiber – consists of a very small glass core, allowing only

a single ray or mode of light to travel across it. This greatly reduces

the attenuation and dispersion of the light signal, supporting high

bandwidth over very long distances, often measured in kilometers.

• Multimode fiber – consists of a larger core, allowing multiple modes

of light to traverse it. Multimode suffers from greater dispersion than

singlemode, resulting in shorter supported distances.

Singlemode fiber requires more precise electronics than multimode, and thus

is significantly more expensive. Multimode fiber is often used for high-speed

connectivity within a datacenter.

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