How to specify root (/) location in web.config?

Try this one:

<system.web>
    <urlMappings enabled="true">
        <add url="~/" mappedUrl="~/default.aspx" />
    </urlMappings>
    <authorization>
        <allow roles="admin"/>
        <deny users="*" />
    </authorization>
</system.web>
<location path="Default.aspx">
    <system.web>
        <authorization>
            <allow users="*" />
        </authorization>
    </system.web>
</location>