How to give folder wise authentication in asp.net

Assuming you are using Membership[^] and Role[^] Providers, all you need to do is set the appropriate permissions in the folders web.config:

<?xml version="1.0" encoding="utf-8"?>
<configuration>
    <system.web>
        <authorization>
            <allow roles="Admin" />
            <deny users="*" />
        </authorization>
    </system.web>
</configuration>

If you are determined to do it yourself, then have a look at Membership Providers and Role Providers - these are similar in effect to the default ones, but you code them yourself. They are certainly the easiest way to do it, the alternative being to reinvent the wheel in your master page Load event - a lot more work.