Ping and Browse

I have a few PCs on one subnet belonging to a workgroup, there are a couple of other PCs belonging to the same workgroup but on a different subnet.All the PCs are connected via a single hub and are using Windows OS.I can see all the machines on network neighborhood but can only ping the ones on the same subnet.What could be the possible explanation for this?I guess the hub has a common bus; shared bandwidth,and is a single broadcast/collision domain this should allow pinging to all the connected devices.
Thanks!
Thanks!