Domain Mapping Question

We created a WP install on WP 3.0 and have MU set up. Got the plugin installed correctly.

The blog site is companyblogs.com and the first blog we are doing is at companyblogs.com/blog1.

We want from blog1.corporatesite.com to go to companyblogs.com/blog1. We configured everything in the backend of companyblogs.com/blog1 making the changes in the plugin screen and on the super admin menu.

The blog1.corporatesite.com is on the same IP as the companyblogs.com right now but we can't get the blog to show up. Getting a 404 error.

It is on a dedicated IP. Can't figure out what else we need to configure or may have missed.