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Unformatted text preview: called the country
domains, or the geographical domains.
Figure 14.2 lists the normal classification of the seven generic domains.
edu file:///D|/Documents%20and%20Settings/bigini/Docu.../homenet2run/tcpip/tcp-ip-illustrated/dns_the.htm (2 of 18) [12/09/2001 14.47.06] Chapter 14. DNS: The Domain Name System gov
org other U.S. governmental organizations
Figure 14.2 The 3-character generic domains. DNS folklore says that the 3-character generic domains are only for U.S. organizations, and the 2-character country
domains for everyone else, but this is false. There are many non-U.S. organizations in the generic domains, and many
U.S. organizations in the .us country domain. (RFC 1480 [Cooper and Postel 1993] describes the .us domain in
more detail.) The only generic domains that are restricted to the United States are .gov and .mil.
Many countries form second-level domains beneath their 2-character country code similar to the generic domains:
.ac.uk, for example, is for academic institutions in the United Kingdom and .co.uk is for commercial
organizations in the United Kingdom.
One important feature of the DNS that isn't shown in figures such as Figure 14.1 is the delegation of responsibility
within the DNS. No single entity manages every label in the tree. Instead, one entity (the NIC) maintains a portion of
the tree (the top-level domains) and delegates responsibility to others for specific zones.
A zone is a subtree of the DNS tree that is administered separately. A common zone is a second-level domain,
noao.edu, for example. Many second-level domains then divide their zone into smaller zones. For example, a
university might divide itself into zones based on departments, and a company might divide itself into zones based on
branch offices or internal divisions.
If you are familiar with the Unix filesystem, notice that the division of the DNS tree into zones is similar to the division
of a logical Unix filesyste...
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