om, ftp. company com, and smtp . company com. A number of types of DNS record types are defined. The DNS records used to resolve a host name into an IP address are called address records, or A records. Other types of records include CNAME (or canonical name or alias records), MX (or mail exchange records), SRV (or Service records, used by SIP and other SIP and the Internet Table 1.1 Internet Top-Level Domains Domain Description Company Network Internet Not-for-profit organization University or college U.S. government U.S. military ARPAnet Information Business museum Museum Professional Air transport industry Cooperatives protocols), and TXT (or free-form text records). Another type of DNS record is a PTR, or pointer record, used for reverse lookups. Reverse lookups are used to map an IP address back to a domain name. These records can be used, for example, in generating server logs that show not only the IP addresses of clients served, but also their domain name. Web browsing provides an example of the use of the DNS system. Another type of DNS record is known as a Naming Authority Pointer (NAPTR) record that can be used by a protocol known as ENUM [14] to map global telephone numbers into Internet URLs. When a user types in a Web address, such as www.artech house. com, the name must be resolved to an IP address before the browser can send the request for the index Web page from the Artech House Web server. The Web browser first launches a DNS query to the IP address for its DNS server, which has been manually configured or set up using DHCP. If the DNS server happens to have the name's A record stored locally (cached) from a recent query, it will return the IP address. If not, the DNS root server will then be que- ried to locate the authoritative DNS server for Artech House, which must con- tain the A records for the artech-house. com domain. The HTTP GET SIP: Understanding the Session Initiation Protocol request is then sent to that IP address, and the Web browsing session begins. There is only one authoritativ