Internal domain name seen as an american domain

Hi all,

I deployed Piwik to make usage statistics for company internal websites and I noticed that visitor locations are kind of "wrong".

They all have an internal IP like 10.0.0.0/8 and they all have a hostname followed by a .net domain which is internal and doesn't exist on the Internet.

Piwik determined that the visitors came from the US, probably due to the .net domain.
How can I specify that .net domain is internal or from my country ?

Thanks for your help
Phil.