và có lẽ các hành vi khác cũng nên được khác nhau cho các ô tô khác nhau. Chúng ta có thể thêm một lớp mới gọi là SportsCar sẽ được thừa hưởng từ ô tô, và chúng tôi có thể cung cấp cho các lớp SportsCar một phương pháp tăng tốc () khác nhau. | 128 NETWORKING CHAP. 7 Furthermore these pipes are typically much larger some may be more than a foot in diameter and because they connect many homes have the capacity to deliver more water than the pipes in your home. Computer networks that most people use on a daily basis appear to span the globe. In reality most computer networks consist of smaller networks which are in turn connected together to form larger networks. An internetwork or Internet is formed when two networks are connected together. In an internet the networks are not connected directly but instead are connected using a computer that is connected to each of the individual networks. This common machine is referred to as a gateway or router and passes information between the two networks. In this chapter we will take a look at computer networks. We will learn how they are organized how they work and some of the applications they can provide. REFERENCE MODEL As with any complicated technology networks are often divided into a number of layers in order to make it easier to understand how they work and to make them easier to build. Each layer is responsible for a different part of the communication process. One of the advantages of using layers is that in order to use a layer you do not need to understand how it works inside you simply need to know what services it provides and how to ask for them. For example consider making a call on a cell phone. You only need to know how to make the call and how to speak into the phone. You do not have to understand the technical mechanisms that convert your voice into a form suitable for transmission using public airwaves. Several reference models have been developed to define a standard way to split the functionality of a network into a series of layers. This layered approach has resulted in casual talk of the network protocol stack The reference model most commonly used in networking was developed by the International Standards Organization ISO and is called the
