Authentication through web.config in ASP.net 3.5

 add this on web.config file.

<authentication mode="Forms">

        <forms loginUrl="default.aspx">

          <credentials passwordFormat="Clear">

            <user name="a" password="b" />

            <user name="c" password="d" />

          </credentials>

        </forms>

      </authentication>

=======================

 <h3>Login</h3>

  <asp:Label id="Msg" ForeColor="maroon" runat="server" /><br />

  Username: <asp:Textbox id="UsernameTextbox" runat="server" /><br />
  Password: <asp:Textbox id="PasswordTextbox" runat="server" TextMode="Password" /><br />

  <asp:Button id="LoginButton" Text="Login" OnClick="Login_OnClick" runat="server" />

========================

public void Login_OnClick(object sender, EventArgs args)
    {
        if (FormsAuthentication.Authenticate(UsernameTextbox.Text, PasswordTextbox.Text))
            Msg.Text = "done";
        else
            Msg.Text = "Login failed. Please check your user name and password and try again.";
    }