A multi site on one core base installation has several advantages, for example:
- Easier maintenance.
- A quick way to set-up multiple sites with the same basic features.
- And what ever other reason you might have.
First, download Drupal 7 and upload it to your web server or shared hosting as usual.
Set-up the first website in the usual way.
Now comes the tricky part.
For the second and following websites you have to create a folder in the sites folder with exactly the same name as the domain name that will be used. For example: for the domain syzygydesign.com you create a folder with the name syzygydesign.com. Copy the file default.setting.php from stites/default to sites/syzygydesign.com and rename it to settings.php and change the permissions so it is writable.
Now you have to connect the domain to your site. In my case (1and1) I had to point the domain name to a folder. You have to use the folder the first website is in!, since the domain name for the second site (syzygydesign.com) and the folder name in the sites folder (syzygydesign.com) are the same Drupal takes care of the rest!
Thereafter you can go to your second site and install Drupal the normal way.
You could use one database for all sites and use a prefix to distinguish between the sites but there is no real need to do so.
Shared modules and themes
All the modules and themes in the folders sites/themes and sites/modules are shared with all websites. (I assume the libraries also but I have not tried it yet)
Sub themes created with Omega 3 end up in the folder sites/themes.
For modules and themes specific for one of the web sites you can create folders in every site directory, for example syzygydesign.com/themes and syzygydesign.com/modules. Themes and modules uploaded to those folders can only be used on that specific domain.
I will go on playing with this set-up and keep you informed.