As is the case in searching a directory, requests in the DNS system can be absolute or relative: fully qualified domain name requests are absolute, and hostname requests are relative to the domain and subdomain (in other words, context) where the client host is located. The operating system will usually construct the queries to conform to the absolute format, which is what DNS servers expect to receive. Relative names must be unique within the boundaries of the subdomain or domain where they are registered (as in directories).
A fully qualified domain name, contrary to the hostname, includes all parent domains up to and including the root domain (as shown in Figure 2-2), which in turn guarantees their uniqueness in the DNS namespace. A prominent example is the "www" hostname. There are probably millions of hosts named www around the world, yet their FQDNs are unique because they include the entire path within the namespace, starting with the hostname and working its way up through the SLD and the TLD to the root.
In the days of Windows NT 4.0, the NetBIOS name and DNS hostname were not related to or dependent on one another. To ensure backward compatibility of applications in Windows 2000/2003/XP and also to reduce administrative overhead, hostnames and NetBIOS names are being set identically. This means that the hostname portion of FQDNs must conform to NetBIOS name requirements. Assigning the same NetBIOS name and DNS hostname is not mandatory, but it is recommended if your network still uses applications that rely on NetBIOS names. You must be sure in this case that you assign valid NetBIOS names that are compatible with host-naming RFCs applicable to DNS. Microsoft recommends combining two namespaces (assigning identical names) or getting rid of NetBIOS entirely, whenever possible.
Valid hostnames in the case of coexistence of both namespaces are those consisting of letters and numbers. The first character must not be a number, and the total hostname length should not exceed 15 characters.
DNS name structure
Underscores are permitted in the NetBIOS namespace, but they are not a valid DNS hostname character. Underscores in DNS names are permitted only in service records, not host records (DNS RFCs: 1034, 1035, 1123, 1995, 1996, 2136, 2181, 2308, and some others). So if you have existing NetBIOS names with underscores, they will have to be changed to hyphens. Some recent revisions of the DNS standard also made it possible to use languages other than English when naming hosts; however, this technology is not very reliable outside the local area network.
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