The host name portion of a URL may actually be an alias, also referred to as a canonical name. In the address http://www.microsoft.com, www is an alias commonly used for World Wide Web servers, Microsoft is the domain name, and .com indicates the commercial branch of the DNS hierarchy of names for the Internet.
The CNAME resource record is used to establish an alias name in DNS server zone files. CNAMEs are frequently used in conjunction with Web, FTP, and Gopher servers and when a host name is changed. The following figure illustrates the New Resource Record dialog box used to create CNAME records.
Figure 2.6 Adding a CNAME record
In the preceding illustration, an alias of www is being associated to a host name. The use of CNAMEs is accepted on the Internet for generalized names for servers such as www to indicate a Web server. However, other uses of the CNAME records can create problems for DNS name resolution throughout the Internet.
RFC 1912, which describes common errors in the creation of DNS resource records, states: "Don't use CNAMEs in combination with RRs (that is, resource records) which point to other names like MX, CNAME, PTR and NS." For more information on these resource record types, see Table 2.1, "DNS Resource Record Types," earlier in this chapter.