Domain Mapping produces reverse effect

I am using WP 3,01, Multi-site, Fixed IP Address, sub-folders, and multiple domain mapping.

In Tools, Domain Mapping, I set (leave) the Original Blog Address = http://mytopblog.com

I set Custom Domain = http://mytopblog.com/mysubblog/

Now, when I go to my "Original Blog Address" in my browser, it takes me to my Custom Domain. I can no longer browse to my original domain.

What is wrong?

Thanks,

Ed