 protocol are then passed to the Internet layer. The Internet layer then passes the packet to the physical layer along with a media access control (MAC) address for routing, in the case of a LAN. A response is processed by reversing the above steps. A response received at the physical layer flows back up the layers, with the header information being stripped off and the response data passed upwards towards the user. The main difference is that no utility is used in response processing. 1.6 DNS and IP Addresses Domain Name Service [13] is used in the Internet to map a symbolic name (such as www.amazon.com to an IP address (such as 100.101.102.103). DNS is also used to obtain information needed to route e-mail messages and, in the future, SIP messages. The use of names instead of numerical addresses is one of the Internet's greatest strengths because it gives the Internet a human, friendly feel. Domain names are organized in a hierarchy. Each level of the name is sepa- rated by a dot, with the highest level domain on the right side. (Note that the dots in a domain name have no correspondence to the dots in an IP address written in dotted decimal notation.) General top-level domains are shown in Table 1.1 (see http://www.icann.org/tlds for the latest list). There is also a set of country domains such as: us (United States), uk (United Kingdom), ca (Canada), and au (Australia). Each of these top-level domains has just one authority that assigns that domain to a user or group. Once a domain name has been assigned, the authority places a link in their DNS server to the DNS server of the user or group who has been assigned the domain. For example, when company. com is allocated to a company, the authoritative DNS server for the top-level com domain entry for company con- tains the IP address of the company's DNS server(s). A name can then be further qualified by entries in the company's DNS server to point to individual servers in their network. For example, the company's DNS server may contain entries for www.company.c