C# Dynamically Changing Master Pages

You fetch the user's IP address and based on his/her country you redirect the user to a specific webpage. Now, how do you change the master page dynamically? This is how I am redirecting the user :-

Geolocation Error with IP Address 127.0.0.1

It's not like the user clicks some button or something and you then change the master page. I want it changed when the user is redirected, so how exactly do I go about it?

public partial class testClass: System.Web.UI.MasterPage
{
protected void Page_Load(object sender, EventArgs e)
{
if (redirected to site1)
{
Use this master page1
}
else
if (redirected to site2)
{
Use this master page2
}
}
}

So how do I check what SITE the user has been redirected to? Also HOW to apply the specific master page now that the user has been redirected?

I just need an idea how to go about it.

[EDIT] please check the code block below. How do I fetch the URL that the user has been redirected to? I actually need just the "iso3166TwoLetterCode" variable's value (see the link to my earlier question, please) and based on that will be changing the master page. I can't figure out how to fetch that value or even use that class (that's got this variable) in my testClass.

protected void Page_PreInit(object sender, EventArgs e)
{
if (user Has been redirected to www.site.in )
{
this.MasterPageFile = "master1.master";
}
if (user Has been redirected to www.site.fr )
{
this.MasterPageFile = "master2.master";
}
}