ALTER TABLE `visitors`
  ADD COLUMN `last_port` VARCHAR(16) DEFAULT NULL AFTER `last_ip`,
  ADD COLUMN `previous_ip` VARCHAR(64) DEFAULT NULL AFTER `last_port`,
  ADD COLUMN `previous_port` VARCHAR(16) DEFAULT NULL AFTER `previous_ip`,
  ADD COLUMN `ip_changed_at` DATETIME DEFAULT NULL AFTER `previous_port`,
  ADD COLUMN `port_changed_at` DATETIME DEFAULT NULL AFTER `ip_changed_at`;