Problems creating a subsite of a subsite

I have a fresh install of Sharepoint 2010 Server Standard.  Out of the box, I cannot create a subsite of a subsite.  I can create subsites with no problem, but if I am at the subsite level, and go to Site Actions --> New Site, I am forced by the url presented to add the new site to http://mainsite/_____________.  What I want to do is create http://mainsite/subsite/subsite.  I can create mainsite/subsite1, mainsite subsite2, etc. but only one subsite deep.  I know this is possible and probably have to enable a feature to get this result, but I cannot find any references to this issue.

Thanks in advance.