<location path="a.aspx">
    <system.web>
        <authorization>
            <allow users="admin" />
            <deny users="*" />
        </authorization>
    </system.web>
</location>

<location path="b.aspx">
    <system.web>
        <authorization>
            <allow users="admin" /> 
            <deny users="*" />
        </authorization>
    </system.web>
</location>

    <system.web>
    ...
        <authentication mode="Forms">
            <forms name="ManageUsers" timeout="5">
                <credentials passwordFormat="MD5">
                    <user name="admin" password="4cfdc2e157eefe6facb983b1d557b3a1" />
                </credentials>
            </forms>
        </authentication>
    ...