Restrict WordPress login by IP address

/**
 * The following will restrict login to a
 * specific IP address.
 *
 * Change "111.111.111.111" to the actual IP.
 */
add_action( 'authenticate', function( $user, $username, $password ) {
    $ip = ( ! empty( $_SERVER['HTTP_CLIENT_IP'] ) ) ? $_SERVER['HTTP_CLIENT_IP'] : $_SERVER['REMOTE_ADDR'];
    if ( '111.111.111.111' != $ip ) {
         $user = new WP_Error( 'authentication_failed', __( '<strong>ERROR</strong>: You are not allowed to log in to that account from this IP.' ) );
    }
    return $user;
}, 100, 3 );