multiple wordpress networks on one server

hi
I would like to set up several multisite installations, however, I only have one IP address at my disposal. now, i want to use Domain Mapping plugin to map multiple domains per installation.each one have its own separate database and admin rights. for example i have two installation of multi site for to different language. site1.com and site2.com.
domain mapping working ok on site1.com but for site2.com it redirect example.site2.com to site1.com signup page

any thoughts? thanks