Domain Mapping - IP configuration

Hi

I have a wordpress 3.0 Multisite network and just installed the domain mapping plug in . i created a a new site with subdirectory

http://www.site1.mydomain.com

When i log on ther bckend of this new site i'm asked to configure the IP of my blog. how can i find the IP adress of my main blog to finish the set up of domain mapping please ?