Networking

6]. In order to understand these layers, we draw a comparison with the postal system (Table 12.1).

Table 12.1 The four network layers of the TCP/IP model

Layer	Network	Postal
Link	The infrastructure of cables, switches, modems and so on which allow information to be moved from one place to another	The infrastructure of vehicles and sorting offices which allow post to be moved around
Network	The addressing system which allows a single computer to be identified and for data to be routed to it	The addressing system which allows post to be directed to the correct destination
Transport	The process of packing the data and sending it into the network. Also the process of receiving data from the network	Taking letters to the post office to send and picking up received letters
Application	Defining the data to be sent and interpreting any data received	Writing and reading your post

Because the network is layered in this way, any changes that occur in one layer do not affect the others, so you do not need to write a new application because you have bought a new modem.

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