 directly the IP address of the destination host. How- ever, that is not feasible in most cases (e.g. the requesting user usually does not know the destination IP address, and it is not convenient to include IPv4 and especially longer IPv6 addresses). DNS is implemented on the application layer (regarding the protocol stack) and ini- tially it used UDP (as a transport protocol) due to its lower delay. However, with the deployment of high-speed Internet links and access around the globe, as well as with the higher processing power in all hosts (which doubles every 1.5-2 years according to Moore's law), there is also a standardized approach for DNS communication by using TCP as transport layer protocol [16]. Overall, proper functioning of DNS as well as lowest possible DNS resolution times are key performance parameters for Internet QoS because they directly refer to Internet service availability and accessibility. 2 Internet QoS 2.1.5 Internet Fundamental Applications 2.1.5.1 Web Technology The World Wide Web (WWW) is a global system started in 1989 by Tim Berners Lee and completed with the development of its main standards during the 1990s. The Web (i.e. WWW) is made up of a large number of documents called web pages, each of which is a hypermedia document. "Hyper" means that it contains links to other websites and "media" means that it can contain other objects besides text (pictures, videos, various files, etc.). The Web is based on the client-server principle. The main protocol for Web end-to- end communication, for access to web pages, is called HTTP. It is based on client-server TCP/IP protocol model, where: Web servers (i.e. HTTP servers) listen on the well-known TCP port 80; Web application on the users' side is called a browser (e.g. Chrome, Firefox, Opera), which has an HTTP client built into it. So, HTTP is only a communication protocol for Web traffic. The presentation of Web content is done with other tools [17], and the most known markup language for creating Web pages is Hypertext Markup Language (H