What is a subnet broadcast?

A broadcast when it applies to networking,means the network delivers one copy of a packet to all the hosts on a network.When it is a subnet broadcast it means the packet is not targeted to all the hosts on the network but only to the hosts that are on a subnet. As for the first part of your question I really can not answer.I do not believe a subnet can end in all 0's like the first subnet that is invalid.I hope a real network guru helps you with that part of your question.I would like to hear the answer myself.