Quite simply, a fully qualified domain name is one that ends in a period. (As opposed to a relative domain name, ending in something else like .com, .net, .org, .edu, .gov, or .(country code).)

It's easy to think of this in terms of directory structure:

Figure 1
/
|
/foo
|  |
|  /bar
|
/baz

Suppose you have a directory foo with a subdirectory bar (fig. 1). If your current directory was foo, you could refer to bar in two ways: "bar" and "/foo/bar"1. The former is relative addressing, the latter is absolute. The same principle holds for domain names.

Figure 2
.
|
.com
|  |
|  .foo
|  |  |
|  |  .bar
|  |
|  .baz
|
.edu

Suppose you have a domain foo.com (fig.2)and a subdomain bar.foo.com. If you are part of the domain foo.com, you can refer to bar in three ways. First, you can simply say "bar" (relative addressing). Secondly, you can say "bar.foo.com" (absolute addressing). Lastly, you can call it "bar.foo.com." (Fully Qualified Addressing). In the last case, it sometimes helps to think of the trailing period like you'd think of the first slash in a directory heirarchy, a sort of "home base".

This can be a particularly nefarious distinction when dealing with DNS Servers: If a server is setup to resolve *.foo.bar to any given IP Address, it will invariably catch any and everything not resolved elsewhere. This is a lazy way to setup things like mail.foo.bar. and ftp.foo.bar., but will also resolve any dead hosts back to yourself. (e.g. if a user types www.mictosoft.com instead of the intended www.microsoft.com, your DNS server will resolve it as www.mictosoft.com.foo.bar.. and direct all traffic to the wrong IP! The way to fix this is to always reference fully qualified domain names in your DNS server configuration.

For more information concerning DNS, Check out O'Reilly's "DNS and Bind by Albitz and Liu".

1 or "\foo\bar", depending on platform.

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