base. Figure 1.3 shows the DNS structure. This structure is seen also in the DNS names - if we examine for instance the name wwww.example.com.' A complete DNS name like this is called a Fully Qualified Domain Name (FQDN). It consists of the following parts, which can be found in Figure 1.3. root is noted by the final dot ('.') at the end of an (FQDN). Root is the top of the hierarchy, and is the central point of the (DNS) database. For the user, this final dot is usually hidden, and the user usually sees 'www.example.com rather than 'www.example.com. Top Level Domain (TLD) is, as the name suggests, the highest point of the domain names. They are, for example, the com, org, and de at the end of a domain name. There Introduction example esempio beispiel ejemplo Figure 1.3 DNS structure. are three categories of TLDs - generic Top Level Domains (gTLDs), country code Top Level Domains (ccTLDs), and an infrastructure TLD. TLDs are more closely examined in Section 1.6.3. An organization, which operates and owns a TLD is called a registrar. Registrars sell or distribute 2nd level domains to the organizations that need domain names. 2nd level domains are owned and controlled by registrars. Registrars include companies, organizations, and even private persons. A 2nd level domain owner can create 3rd level domain names, and distribute those in the way the 2nd level domain owner wishes. 3rd or lower level domain are domains created under the 2nd level. They can indicate a service (such as www, ftp, etc.) or they can be otherwise descriptive - for instance CS. helsinki. fi is the domain name of the computer science department of the University of Helsinki. 1.6.2 DNS Operation The previous section explained the structure of the DNS name. This section will now examine how the actual DNS resolution - mapping a DNS name to an IP address - works actually. Figure 1.4 shows a simplified overview of the process, and in the following we examine the resolution, step by step: 1. The process starts when a host has an FQDN that it wishes 