While the
OSI model distinguishes the services to be provided by each of these layers, this portion of the OSI model has
never been widely adopted in practice.
Instead, the highest layers of the protocol stack have been joined together as the application layer. The rationale
has been that once the message has been reliably transmitted between computers, possibly via a world wide
web of computer connections, understanding the meaning of the message, and reacting appropriately, is the
responsibility of whatever application program received the message. This approach has become known as the
internet reference model, and it is shown in Fig. 7-2. The internet reference model is the most widely used
model in practice.
CHAP. 7] NETWORKING 129
Figure 7-2 Internet reference model.
The internet model contains only four layers, yet the widely used internet model also can be mapped to the
more general OSI model:
1 The subnet (data link) layer of the internet model takes on the responsibility of both the physical and data
link layers of the OSI model. In most computers, this layer is implemented by the firmware on the network
interface card and the corresponding drivers in the operating system.
2 The internet (network) layer of the internet model provides computer-to-computer delivery using the
internet protocol (IP), and it corresponds to the network layer of the OSI model.
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