network broadcast to or x.x.x.255 doesn't work

I'm using the ics UDPclinet / server example to attempt to find
machines on the network that are UDP servers at the particular port.
It's fine when I specify the particular port address, but using either
the network mask X.X.X.255 or, none of the servers
pick up the clinet send message.
What is wrong or necessary to fix it?