I can't find the reference, but I think there was recently some general discussion of www.example.com/widgets/ vs. widgets.example.com, and the subdirectory approach was favored from the standpoint of SE performance.
Looking specifically at Google, a subdomain will be a different site, and hence will have its own PR rather than being part of a single site's integrated PR structure.
Overall, I don't think a forum is a substantially different situation than another topic. However, if a forum is successful, it may end up dwarfing the rest of the site in size and traffic.
I made the one forum I run a sub-domain. I did this primarily for simplicity sake - i.e. the URIs for the forum were far too messy for my liking. I also had in mind the SEs and wanted to see how they'd read/rank it.
I did subdomain because it had enough content and activity that people wanted to go to it easily enough. In return it got listed in directories seperate than my main site because it offered great content, and I called the forum another name, that is what it got listed under.
The sub-domain option allows for easier moving to a different server if you ever wanted to.
I had my forum set up as a subdomain (which I preferred), but then I wanted to extend the functionality of the registration and login system to my entire site (one login for forum & member area) - it turned out the easiest way to do this was to move the forum to a subdirectory to avoid "root path" and redirection conflicts. You may want to consider the capabilities/flexibility of the forum software you're using in your decision process. :)
thank you all for your valuable hints and information.
I will now start the forum in a sub-directory to help it from the main page's PR-context in its early days. If it will grow and develop properly, then I will probably move it to a sub-domain or even an own domain later.
The forum will be based on YaBB, so it should be easy to move it and just change the forum's domain/link/path configuration and set some redirects.
Thanks again and regards,
A subdomain allows a much more memorable url most people will goto a homepage and then click a link to the forum if the forum is located as
than if the forum is located at
it is specific to your own website if you want all traffic entering the site via the homepage mayb because of statistics or because adverts are based around the homepage rather than forum
personaly i feel that
[kb.example.com...] -knowledge base
looks much more professional and takes people to exactly where they want to go
I kind of like the subdomain approach too, as a popular forum can become a huge percent of site content. But the unified login capability can be important, too. Busynut mentions that a subdomain can complicate this - how difficult are these issues to overcome with a subdomain-based forum?