Linux Network Administrators Guide
Prev
Chapter 6. Name Service and Resolver Configuration
Next
6.2. How DNS Works
DNS organizes hostnames in a domain hierarchy. A domain is a collection of sites that are related in some sense—because they form a proper network (e.g., all machines on a campus, or all hosts on BITNET), because they all belong to a certain organization (e.g., the U.S. government), or because they're simply geographically close. For instance, universities are commonly grouped in the edu domain, with each university or college using a separate subdomain , below which their hosts are subsumed. Groucho Marx University have the groucho.edu domain, while the LAN of the Mathematics department is assigned maths.groucho.edu . Hosts on the departmental network would have this domain name tacked onto their hostname, so erdos would be known as erdos.maths.groucho.edu . This is called the fully qualified domain name (FQDN), which uniquely identifies this host worldwide.
Figure 6-1 shows a section of the namespace. The entry at the root of this tree, which is denoted by a single dot, is quite appropriately called the root domain and encompasses all other domains. To indicate that a hostname is a fully qualified domain name, rather than a name relative to some (implicit) local domain, it is sometimes written with a trailing dot. This dot signifies that the name's last component is the root domain.
Figure 6-1. A part of the domain namespace
Depending on its location in the name hierarchy, a domain may be called top-level, second-level, or third-level. More levels of subdivision occur, but they are rare. This list details several top-level domains you may see frequently:
Domain
Description
edu
(Mostly U.S.) educational institutions like universities.
com
Commercial organizations and companies.
org
Non-commercial organizations. Private UUCP networks are often in this domain.
net
Gateways and other administrative hosts on a network.
mil
U.S. military institutions.
gov
U.S. government institutions.
uucp
Officially, all site names formerly used as UUCP names without domains have been moved to this domain.
Historically, the first four of these were assigned to the U.S., but recent changes in policy have meant that these domains, named global Top Level Domains (gTLD), are now considered global in nature. Negotiations are currently underway to broaden the range of gTLDs, which may result in increased choice in the future.
Outside the U.S., each country generally uses a top-level domain of its own named after the two-letter country code defined in ISO-3166. Finland, for instance, uses the fi domain; fr is used by France, de by Germany, and au by Australia. Below this top-level domain, each country's NIC is free to organize hostnames in whatever way they want. Australia has second-level domains similar to the international top-level domains, named com.au and edu.au . Other countries, like Germany, don't use this extra level, but have slightly long names that refer directly to the organizations running a particular domain. It's not uncommon to see hostnames like ftp.informatik.uni-erlangen.de . Chalk that up to German efficiency.
Of course, these national domains do not imply that a host below that domain is actually located in that country; it means only that the host has been registered with that country's NIC. A Swedish manufacturer might have a branch in Australia and still have all its hosts registered with the se top-level domain.
Organizing the namespace in a hierarchy of domain names nicely solves the problem of name uniqueness; with DNS, a
* License
