how to authenticate active directory users in my asp.net on another server on the same network

Below is the sample for LDAP authentication (you can take more details from http://support.microsoft.com/kb/316748[^]):

Just pass valid Domain name (as you said it should be reachable), you need not to worry whether you application is lying in which domain.


public bool IsAuthenticated(String domain, String username, String pwd)
{
String domainAndUsername = domain + @"\" + username;
DirectoryEntry entry = new DirectoryEntry(_path, domainAndUsername, pwd);


try
{ //Bind to the native AdsObject to force authentication.
Object obj = entry.NativeObject;


DirectorySearcher search = new DirectorySearcher(entry);


search.Filter = "(SAMAccountName=" + username + ")";
search.PropertiesToLoad.Add("cn");
SearchResult result = search.FindOne();


if(null == result)
{
return false;
}


//Update the new path to the user in the directory.
_path = result.Path;
_filterAttribute = (String)result.Properties["cn"][0];
}
catch (Exception ex)
{
throw new Exception("Error authenticating user. " + ex.Message);
}


return true;
}