Thanks to all.
I'm trying connection throguh MAC with MT. Actually, Winbox is showing me both, but... which of them does it use for connecition?.
Would you tell that it could be a version problem?.
Winbox allows you to connect to the MT using both MAC address and ip address. If you type in a MAC address on the address line it will use IPv6 to connect to the MT. If you use a IP address in the address line it will use IPv4 to connect to the MT.
To my knowledge i believe it has to do with the network card in use and the software it runs through. I have had this issue with Windows XP and routerboard 44. I ended up taking the nics off auto neg and things worked so much better and the problem was resolved.