The network layer is concerned with end-to-end communication between computers that are not necessarily
directly connected one to another. When machines that wish to communicate are not directly connected, they
must rely on other machines that are directly connected to relay their messages. The network layer is the lowest
layer of networking protocol with a focus on exchanging messages by means of relaying communications
between intervening computers.
For example, you have probably passed notes in class. If the person to whom you want to pass the note
is on the other side of the classroom, you have to rely on the people sitting between you to pass the message.
It may be the case that you know that your message will be safely exchanged between yourself and the person
sitting next to you (i.e., point-to-point communication), but once the message is in the hands of the next person
in the chain, it may be lost.
We can also use the analogy of passing notes in a classroom to illustrate one of the most important
services provided by the network layer: routing. You may realize that there are several paths by which your
message could make its way to your friend. Some of these paths may be faster than others, or some may be
more reliable.
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