The Application layer [also called layer 5) is the software that enables the user to perform useful work. The software at the application layer is the reason for having the network because it is this software that provides the business value. This topic examines the three fundamental types of application architectures used at the application layer [host-based, client-based, client-server). It then looks at the Internet and the primary software application packages it enables: the Web, e-mail, Telnet, FTP, and Instant Messaging.
■ Understand host-based, client-based, and client-server application architectures
■ Understand how the Web works
■ Understand how e-mail works
■ Be aware of how FTP, Telnet, and instant messaging works
Network applications are the software packages that run in the application layer. You should be quite familiar with many types of network software, because it is these application packages that you use when you use the network. In many respects, the only reason for having a network is to enable these applications.
In this topic, we first discuss three basic architectures for network applications and how each of those architectures affects the design of networks. Because you probably have a good understanding of applications such as the Web and word processing, we will use those as examples of different application architectures. We then examine several common applications used on the Internet (e.g., Web, e-mail) and use those to explain how application software interacts with the networks.