Getting base domain url

How do I get the base URL of a multiuser site with subdomains?
For example, on a site with the domain http://site.com I would use bloginfo(‘url’)
This would take me to http://site.com.

Lets say I’m on a subdomain like http://blog.site.com
Then the code would take me to http://blog.site.com.
What I need is to make it go to http://site.com