Ì mean which offical servers such as kRo/fRO/iRO/japaneseRO are free to play? I tried to google it but google wont help me......
Well normally all of official servers got a free to play server, as for example iRO got Valkyrie which is official- and free. iRO's Ymir and Yggdrasil server (which previously were pay-to-play) are going to become free of charge as well.
http://irowiki.org/wiki/F2P (http://irowiki.org/wiki/F2P)
There's a chart stating the official free to play servers along with additional information to compare each. /no1