The cloud symbol suggests another network, perhaps the entire
Internet. It reminds us that there is a way to connect to that other
network (the Internet), but does not supply all the details of either
the connection or the network.
The physical features of the cloud are many. To help you
understand, you might think of all of the devices that connect your computer to some very
distant computer, perhaps on another continent. There is no single picture that could
display all of the processes and equipment that would be involved in making that
connection. 
The purpose of the cloud is to represent a large group of
details that are not pertinent to a situation, or description, at a given time.
It is important to remember that at this point in the curriculum, you
are only interested in how LANs connect to larger WANs and to the
Internet (the ultimate WAN), so that any computer can talk to any other
computer, any place and any time. Because the cloud is not really a single device, but a collection of devices that operate at all levels of the OSI model, it is classified as a Layer 1-7 device.
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