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mr.ccie
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افتراضي رد: The TCP/IP and OSI بالصور شرح رائع وممتع


Foundation Topics
It is practically impossible to find a computer today that does not support the set of
networking protocols called TCP/IP. Every Microsoft, Linux, and UNIX operating system
includes support for TCP/IP. Hand-held digital assistants and cell phones support TCP/IP.
And because Cisco sells products that create the infrastructure that allows all of these
computers to talk with each other using TCP/IP, Cisco products also include extensive
support for TCP/IP.
The world has not always been so simple. Once upon a time, there were no networking
protocols, including TCP/IP. Vendors created the first networking protocols; these protocols
supported only that vendor’s computers, and the details were not even published to the
public. As time went on, vendors formalized and published their networking protocols,
enabling other vendors to create products that could communicate with their computers.
For instance, IBM published its Systems Network Architecture (SNA) networking model
in 1974. After SNA was published, other computer vendors created products that allowed
their computers to communicate with IBM computers using SNA. This solution worked,
but it had some negatives, including the fact that it meant that the larger computer vendors
tended to rule the networking market.
A better solution was to create an open standardized networking model that all vendors
would support. The International Organization for Standardization (ISO) took on this task
starting as early as the late 1970s, beginning work on what would become known as the
Open System Interconnection (OSI) networking model. ISO had a noble goal for the OSI
model: to standardize data networking protocols to allow communication between all
computers across the entire planet. ISO worked toward this ambitious and noble goal, with
participants from most of the technologically developed nations on Earth participating in
the process.
A second, less formal effort to create a standardized, public networking model sprouted
forth from a U.S. Defense Department contract. Researchers at various universities
volunteered to help further develop the protocols surrounding the original department’s
work. These efforts resulted in a competing networking model called TCP/IP.
By the late 1980s, the world had many competing vendor-proprietary networking models
plus two competing standardized networking models. So what happened? TCP/IP won in
the end. Proprietary protocols are still in use today in many networks, but much less so than
in the 1980s and 1990s. The OSI model, whose development suffered in part because of a
slower formal standardization process as compared with TCP/IP, never succeeded in the
marketplace. And TCP/IP, the networking model created almost entirely by a bunch of
volunteers, has become the most prolific set of data networking protocols ever.
22 Chapter 2: The TCP/IP and OSI Networking Models
In this chapter, you will read about some of the basics of TCP/IP. Although you will
learn some interesting facts about TCP/IP, the true goal of this chapter is to help you
understand what a networking model or networking architecture really is and how one
works.
Also in this chapter, you will learn about some of the jargon used with OSI. Will any of
you ever work on a computer that is using the full OSI protocols instead of TCP/IP?
Probably not. However, you will often use terms relating to OSI. Also, the ICND1 exam
covers the basics of OSI, so this chapter also covers OSI to prepare you for questions about
it on the exam.
The TCP/IP Protocol Architecture
TCP/IP defines a large collection of protocols that allow computers to communicate.
TCP/IP defines the details of each of these protocols inside documents called Requests for
Comments (RFC). By implementing the required protocols defined in TCP/IP RFCs, a
computer can be relatively confident that it can communicate with other computers that also
implement TCP/IP.
An easy comparison can be made between telephones and computers that use TCP/IP.
You go to the store and buy a phone from one of a dozen different vendors. When you get
home and plug in the phone to the same cable in which your old phone was connected,
the new phone works. The phone vendors know the standards for phones in their country
and build their phones to match those standards. Similarly, a computer that implements
the standard networking protocols defined by TCP/IP can communicate with other
computers that also use the TCP/IP standards.
Like other networking architectures, TCP/IP classifies the various protocols into different
categories or layers. Table 2-2 outlines the main categories in the TCP/IP architectural
model.
The TCP/IP model represented in column 1 of the table lists the four layers of TCP/IP,
and column 2 of the table lists several of the most popular TCP/IP protocols. If someone
Table 2-2
TCP/IP Architectural Model and Example Protocols

TCP/IP Architecture Layer Example Protocols
Application HTTP, POP3, SMTP
Transport TCP, UDP
Internet IP
Network access Ethernet, Frame Relay
The TCP/IP Protocol Architecture 23
makes up a new application, the protocols used directly by the application would be
considered to be application layer protocols. For example, when the World Wide Web
(WWW) was first created, a new application layer protocol was created for the purpose of
asking for web pages and receiving the contents of the web pages. Similarly, the network
access layer includes protocols and standards such as Ethernet. If someone makes up a
new type of LAN, those protocols would be considered to be a part of the network access
layer. In the next several sections, you will learn the basics about each of these four layers
in the TCP/IP architecture and how they work together
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