Deny link in ASP.net ?

Hello guys.I have problem as follow , i will checked "group" and not access some link in ASP.net .

Example : Submenu1
                   +Link 1.aspx
                   +Link 2.aspx
                Submenu2
                   +Link 3.aspx
                   +Link 4.aspx
Now i want to checked , if user typing url in web browser, it will deny and redirect a link orther .
Example: User have "group A" only access Link 1.aspx and Link 4.aspx. If user typehttp://contoso.com/link2.aspx , it will deny and redirect page Default.aspx. User have "group B" only access Link 2.aspx , if user typehttp://contoso.com/link1.aspx , it will deny and redirect page Default.aspx,...etc.

Can you give me for some advice.Thank you.