Thanks for your reply and your suggestion,
in any case, even just to learn something, I would like to investigate the possibility to use still direclty the arduino. I haven't problem with the speed, because I'm thinking just to a portable device to help in the understanding of the routing in a complex network, with test rate very very low (few packets per minute). About the promiscous mode: do you mean that even the UDP packets with "wrong" destination address are dropped by the wiznet? I was thinking that such kind of processing is done in higher level of the stack...
g.