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analysis of the wide-area network traffic generated by the DNS is given in [Danzig, Obraczka, and Kumar 1992]. 14.2 DNS Basics
The DNS name space is hierarchical, similar to the Unix filesystem. Figure 14.1 shows this hierarchical organization. file:///D|/Documents%20and%20Settings/bigini/Docu.../homenet2run/tcpip/tcp-ip-illustrated/dns_the.htm (1 of 18) [12/09/2001 14.47.06] Chapter 14. DNS: The Domain Name System Figure 14.1 Hierarchical organization of the DNS.
Every node (circles in Figure 14.1) has a label of up to 63 characters. The root of the tree is a special node with a null
label. Any comparison of labels considers uppercase and lowercase characters the same. The domain name of any node
in the tree is the list of labels, starting at that node, working up to the root, using a period ("dot") to separate the labels.
(Note that this is different from the Unix filesystem, which forms a pathname by starting at the top and going down the
tree.) Every node in the tree must have a unique domain name, but the same label can be used at different points in the
A domain name that ends with a period is called an absolute domain name or a fully qualified domain name (FQDN).
An example is sun.tuc.noao.edu.. If the domain name does not end with a period, it is assumed that the name
needs to be completed. How the name is completed depends on the DNS software being used. If the uncompleted name
consists of two or more labels, it might be considered to be complete; otherwise a local addition might be added to the
right of the name. For example, the name sun might be completed by adding the local suffix .tuc.noao.edu.. The
top-level domains are divided into three areas:
1. arpa is a special domain used for address-to-name mappings. (We describe this in Section 14.5.)
2. The seven 3-character domains are called the generic domains. Some texts call these the organizational
3. All the 2-character domains are based on the country codes found in ISO 3166. These are...
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