404. That’s an error.
The requested URL /bookmarks/mark?op=add&bkmk=https%3A%2F%2Fru.adminsub.net%2Fipv4-subnet-calculator&title=%D0%91%D0%B5%D1%81%D0%BF%D0%BB%D0%B0%D1%82%D0%BD%D1%8B%D0%B9+%D0%BE%D0%BD%D0%BB%D0%B0%D0%B9%D0%BD-%D0%BA%D0%B0%D0%BB%D1%8C%D0%BA%D1%83%D0%BB%D1%8F%D1%82%D0%BE%D1%80+%D0%BF%D0%BE%D0%B4%D1%81%D0%B5%D1%82%D0%B8+IPv4+-+ru.adminsub.net
was not found on this server. That’s all we know.