QUdpSocket & broadcast & Windows7

I am sending a request (broadcast: 255.255.255.255) by UDP to determine who is where in the network. The program works fine under WinXP, Ubuntu but under Windows7 sent to any address other than broadcast (255.255.255.255). Other programs written in C # and VB work. Prompt in what direction to dig.

I then found out that is not sent because the IP is absent. Accordingly, the 255.255.255.255 does not send the same reason. Strange did not UDP should still send?