LDAP Authentication in ASP.Net c#

in config file ..

  <authentication mode="Forms">

              <forms loginUrl="~/Account/LogOn" timeout="10"/>

        </authentication>                  

in code behind

 public  ValidateUser(string user, string pass)

        {

            bool validation;

            try

            {

                LdapConnection ldc = new LdapConnection(new LdapDirectoryIdentifier((string)null, false, false));

                NetworkCredential ncon = new NetworkCredential(user, pass, "DOMAINNAME");

                ldc.Credential = ncon;

                ldc.AuthType = AuthType.Negotiate;

                ldc.Bind(ncon); 

                validation = true;

            }

            catch (LdapException)

            {

                validation = false;

            }

            return validation;

        }