FormsAuthentication.RedirectFromLoginPage
is used to send the user to whatever URL they were trying to get to directly. For example, if I am a user of your site and I try to browse to http://yoursite/somepage.aspx if I have not been authenticated then .Net will send me to your login page. After you authenticate my login if you call
FormsAuthentication.RedirectFromLoginPage
I will be sent to somepage.aspx. If you always want them to go to default.aspx then Response.Redirect them instead.