The term Internet Exchange Point is defined in Article 4(13) and clarified further in recital 18 and can be described as a network facility that enables the interconnection of more than two independent technically stand-alone systems, with the primarily purpose to facilitate the exchange of internet traffic. The Internet Exchange Point can also be described as a physical location where a number of networks can exchange internet traffic with each other via a switch. The primary purpose of an IXP is to allow networks to interconnect directly, via the exchange, rather than through one or more third-party networks. The IXP provider is normally not responsible for the routing of the internet traffic. The rooting of the traffic is done by the network providers. The advantages of the direct interconnection are numerous, but the primary reasons are cost, latency, and bandwidth. Traffic passing through an exchange is typically not billed by any party, whereas traffic to an upstream Internet Service Provider (ISP) is. The direct interconnection, often located in the same city as both networks, avoids the need for data to travel over long distances to get from one network to another, thus reducing latency. It should be noted that the definition of IXP does not cover physical points where only two physical networks interconnect with each other (i.e. the network providers such as BASE and PROXIMUS). Therefore when transposing the Directive Member States must differentiate between operators who are facilitating the exchange of aggregated internet traffic between multiple network operators and those who are single network operators, which physically interconnect their networks based on an interconnection agreement. In the latter case, the network providers are not covered by the definition in Article 4(13). A clarification on this matter can be found in recital 18 which states that the IXP does not provide network access or act as a transit provider or carrier. The last category of providers are undertakings providing public communications networks and/or services which are subject to the security and notification obligations of Article 13a and 13b of Directive 2002/21/EC and therefore excluded from the scope of the NIS Directive.26 2) Domain Name System (DNS). The term domain name system is defined in Article 4(14) as "a hierarchical distributed naming system in a network which refers queries for domain names". More precisely, the DNS can be described as a hierarchical distributed naming system for computers, services or any other resource connected to Internet which enables the encoding of domain names into IP (Internet Protocol) addresses. The main role of the system is to translate the assigned domain names into IP addresses. For this purpose, DNS is operating a data base and using name servers and resolver to enable this kind of "translation" of the domain names into operational IP addresses. Although the encoding of domain names is not the only one responsibility of the DNS, it is a core task of the system. The legal definition in Article 4(14) focuses on the main role of the system from the user's point of view without going into more technical details, as for example the operation of domain name space, name servers, resolvers, etc. Finally, Article 4(15) clarifies who is to be considered as a provider of DNS services. 26 See section 5.2. for more details on the relationship between the NIS Directive and Directive 2002/21/EC 21