I have a LAN of windows machine some running XP some running 2000, my broadcast ping times out all the time.
is it due to firewall? or some other setting in registry?
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Windows boxes don't respond to broadcast pings and depending on the distro quite a few Linux boxes don't either. IIRC the same applies to BSD server defaults. It's not something you generally want systems to do, certainly not recently. The problem is not with the system you are sending the broadcast out from but from the remote systems ignoring it.
There may well be a registry key that can modify this (EnableBcastArpReply under the TCPIP service key looks like a likely suspect) but it would need to be changed on all the targets which makes the exercise a bit pointless if you are using the broadcast to discover systems.