Security :: Windows Authentication / "Permission Denied" Error?

I am writing an internal web app for a company who wont to use their existing Active Directory Logins (rather than a seperate SQL Database User Login Table).

To do this I have done the following

web.config
<authentication mode="Windows">
<identity impersonate="true"/>
<authorization>
<deny users="?"/>
<!-- Allow Public Users to Access -->
<allow users="*"/>
</authorization>
<roleManager enabled="true"
defaultProvider="AspNetWindowsTokenRoleProvider"/>
End

This seems to work on my localhost but I have a couple of questions for you guys.

1) How does this process know which active directory to authenticate against? I know in code you can do

LdapAuthentication adAuth = new LdapAuthentication("LDAP://Server");

But there seems to be no way to do this in the web.config (I'm guessing it will just use the active directory on current machine it's running on?). Is there away to specify which active directory I'd like it to authenticate against?

2) If a user tries to access this remotely (i.e. not on their native windows account) what will happen, will it just default to a login form or just throw a "Permission Denied" error?