Forums Connection Issues "unable to resolve server"

i am using a router in my home network
the router is B-Focus 312+
in my computer i cant log in any server with mirc
it always saying "unable to resolve server".
in my other computer, connected to the same router, i can.
the settings in the mirc are the same and i cant fix the problem.
(i dont have any firewall, not window's and not the router's)
please help me understanding the problem.
i will be very greatfull.