Internet has changed a lot since its early days. At the beginning, goal of the Internet was to share computing resources between universities and scientists. In those days communication happened almost completely directly between computers or people. This is something the authors now call peer-to-peer communication. After the commercialization of Internet, millions and millions of home users started to use different services like web and e-mail. This led to a situation where new technologies for addressing and communication had to be developed, client/server model was born.