nternet. However, it is characterized by two conceptually inde- pendent aspects: The first is the definition of names and rules for delegation of authority for the names. The second is specification of a distributed computer system which will provide effi- cient mapping of domain names to IP addresses and vice versa. The domain name space is defined as a tree structure, with the root on the top. Each domain name consists of a sequence of labels, which are separated with dots, and each label corresponds to a separate node in the domain tree. For example, in the domain name "www.itu.int" one will observe that it consists of three labels divided by dots. The label on the right side is always higher in the name hierarchy. So, in the given example the top-level domain is the domain "int." Regarding QoS, DNS availability is very important for Internet services' accessibil- ity. In fact, besides a physical connection to the network, to have availability of Internet access service (as a whole) requires an IP address (which is allocated dynamically with DHCP) and at least one available DNS server (and its IP address). Typically, IP addresses of two or three DNS servers are set up in each host connected to the Internet, due to the importance of the DNS functionalities. Each host has a DNS client built into it, which contacts the DNS server (from a defined list). In order to have lower RTT for DNS resolution (e.g. resolution of a domain name into an IP address), it is important to have DNS servers closer to the clients (e.g. in the same IP network, where possible), although a DNS client can send a request to any available DNS server on the Internet. However, not all DNS servers have records for all possible domain names. If a cer- tain record is not available, the contacted DNS server sends a request to another DNS server either iteratively or recursively (to different DNS servers, according to obtained responses). If there is no possible DNS communication, the user can still connect to a server or a peer host by specifying