> As I stated earlier, A and B and physically next to each other, and
> they share the same cable modem, the same dhcp, same isp, etc.  I
> tried disabling the firewall, but that didn't change anything.

Some "high speed Internet" setups don't work for more than one machine
at a time.  Typically, they are willing to talk with only one MAC
address, needing to be reset to change to another.

With one addition, this theory explains everything you've described.
That addition is the postulation that the cablemodem box itself is
responsible for answering at your "default router" address, and
whatever answers pings there is *not* so limited - but whatever handles
passing packets upstream to the cable head end *is*.

Whether this - assuming it's what's causing your problem - is something
you can actually blame your provider for depends on whether your uplink
is supposed to support multiple directly-connected hosts.  Some are;
some aren't.

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