Because I can never find this post when I'm looking for it:
Copied from:
http://scottwater.com/blog/archive/2006/04/18/CS_Subdomains.aspxCS Subdomains
Timothy Humphrey has release a very nice Community Server add-on which enables you to quickly and easily setup blogs/forums/etc as individual subdomains.
The SubdomainModule class internally redirects
specific urls to specific portions of Community Server; it can also
intercept links before they are rendered and change them to appear to
be within the subdomain url used to access the page. The net effect is
that a portion of Community Server can be isolated from the entire
community and appear to be a standalone application while unaffecting
the rest of the community.
For example, normally blogs
are stored under the blogs/(blog) directory, where (blog) is the blog
name. To access a blog named "personal" you would normally use
http://server.com/blogs/personal. With the SubdomainModule class you
could have a subdomain called "personal" that points to this url and
access it with the following url, http://personal.server.com.
You can download the add-on here.