According to Sun, packages should be namespaced according to the inverse of your domain name, and then followed by whatever you see fit. Most companies or organisations have a .com, or .org domain name, and hence most packages start with com. or org.. To quote from the Sun Code Conventions:
The prefix of a unique package name is always written in all-lowercase ASCII letters and should be one of the top-level domain names, currently com, edu, gov, mil, net, org, or one of the English two-letter codes identifying countries as specified in ISO Standard 3166, 1981.
Subsequent components of the package name vary according to an organization's own internal naming conventions. Such conventions might specify that certain directory name components be division, department, project, machine, or login names.
And the examples that they give, make it pretty clear that you are meant to use the companies DNS name:
com.sun.eng
com.apple.quicktime.v2
edu.cmu.cs.bovik.cheese
You will also see edu. and net. packages out in the wild as well, although they are less common.
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