The Network Edge
In the previous section we presented a high-level overview of the internet and networking protocols. We are now going to delve a bit more deeply into the components of a computer network (and the internet, in particular). We begin in this section at the edge of a network and look at the components with which we are most familiar – namely, the computers, smartphones and other devices that we use on a daily basis. In the next section we’ll move from the network edge to the network core and examine switching and routing in computer networks.
The computers and other devices connected to the internet are often referred to as end systems. They are referred to as end systems because they sit at the edge of the internet. The internet’s end systems include desktop computers, servers (e.g. web and e-mail servers), and mobile computers (laptops, smartphones, and tablets). Furthermore, an increasing number of non-traditional devices are being attached to the internet as end systems.
End systems are also referred to as hosts because they host (that is, run) application programs such as a web browser program, a web server program, an e-mail client program, or an e-mail server program. Throughout this course we will use the terms hosts and end systems interchangeably; that is , host = end system. Hosts are sometimes further divided into two categories: clients and servers. Informally, clients tend to be desktop and mobile PCs, smartphones, and so on, whereas servers tend to be more powerful machines that store and distribute web pages, stream video, relay e-mail , and so on. Today, most of the servers from which we receive search results, e-mail, web pages, and videos reside in large data centres. For example, Google has 30-50 data centres, with many having more than one hundred thousand servers.