Overview of the TCP/IP Networking Model
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The TCP/IP model both defines and references a large collection of protocols that allow
computers to communicate. To define a protocol, TCP/IP uses documents called Requests
For Comments (RFC). (You can find these RFCs using any online search engine.) The TCP/IP
model also avoids repeating work already done by some other standards body or vendor consortium
by simply referring to standards or protocols created by those groups. For example,
the Institute of Electrical and Electronic Engineers (IEEE) defines Ethernet LANs; the
TCP/IP model does not define Ethernet in RFCs, but refers to IEEE Ethernet as an option.
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, when you buy a new computer today, it implements the TCP/IP model to the
point that you can usually take the computer out of the box, plug in all the right cables,
turn it on, and it connects to the network. You can use a web browser to connect to your
favorite website. How? Well, the OS on the computer implements parts of the TCP/IP
model. The Ethernet card, or wireless LAN card, built in to the computer implements some
LAN standards referenced by the TCP/IP model. In short, the vendors that created the hardware
and software implemented TCP/IP.
To help people understand a networking model, each model breaks the functions into a
small number of categories called layers. Each layer includes protocols and standards that
relate to that category of functions. TCP/IP actually has two alternative models, as shown in
Figure 1-4 Two TCP/IP Networking Models
The model on the left shows the original TCP/IP model listed in RFC 1122, which breaks
TCP/IP into four layers. The top two layers focus more on the applications that need to
send and receive data. The bottom layer focuses on how to transmit bits over each individual
link, with the Internet layer focusing on delivering data over the entire path from the
original sending computer to the final destination computer.
The TCP/IP model on the right shows the more common terms and layers used when
people talk about TCP/IP today. It expands the original model’s link layer into two separate
layers: data link and physical (similar to the lower two layers of the OSI model). Also, many
people commonly use the word “Network” instead of “Internet” for one layer.
NOTE The original TCP/IP model’s link layer has also been referred to as the network
access and network interface layer.
Many of you will have already heard of several TCP/IP protocols, like the examples listed
in Table 1-2. Most of the protocols and standards in this table will be explained in more
detail as you work through this book. Following the table, this section takes a closer look at
the layers of the TCP/IP model.
Table 1-2 TCP/IP Architectural Model and Example Protocols