<authentication mode="Forms" />

<system.web>
  <authorization>
    <deny users="?" />
  </authorization>
</system.web>

<system.web>
 ...
 <membership defaultProvider="MembershipADProvider">
  <providers>
    <add
      name="MembershipADProvider"
      type="System.Web.Security.ActiveDirectoryMembershipProvider, System.Web,
            Version=2.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a"
                connectionStringName="ADConnectionString"
                connectionUsername="<domainName>administrator"
                connectionPassword="password"/>
   </providers>
 </membership>
 ...
</system.web>

if (Membership.ValidateUser(userName.Text, password.Text))
{
    FormsAuthentication.RedirectFromLoginPage(userName.Text, false);
}
else
{
  Response.Write("Invalid UserID and Password");
}