1. I'll say it yet again... being on a shared ethernet segment
opens one up to all sorts of nasty problems. If other
customers and/or the ISP have gear in the same subnet, you
are vulnerable.
2. Try pinging the "broken" IP addresses from a machine in the
same subnet. Check the ARP cache to say if ARP resolution is
working.
I'm tempted to say they're routing you a /28 and told you the
wrong subnet mask.
Here's another test:
* Find a host in the same /24 as your IPs
* Try pinging them from the outside world and your machine. If
you can ping from the outside world, AND you can NOT ping from
your machine, AND you do NOT get ARP resolution from your
machine, your subnet mask probably is wrong.