/**
 * Implements hook_form_FORM_ID_alter
 */
function mymodule_form_user_register_alter(&$form, &$form_state) {
  $form['#validate'][] = 'mymodule_user_register_validate';
}

/**
 * hook_form_user_register_alter validation callback
 */
function mymodule_user_register_validate($form, &$form_state) {
  $account = (object) $form_state['values'];
  $ip_address = ip_address(); // api call
  // do something to look up ip_address on registered user..
  // for example:
  $count = db_query("SELECT count(uid) FROM {user_ip} WHERE ip = :ip", array(':ip' => $ip_address))->fetchField();
  if($count > 3){
    $error = t('Sorry, too many registrations.');
    form_set_error('name', $error);
  }
}