IP Address Question

[Dazed_75]

I was working on a friend's systems today and saw something I did not
understand.  Yes, they were windows boxes, but my question is purely a
networking question.  Basically he has a desktop machine with a wired
connection to a wireless AP/router (Airport brand I think) and a
laptop using a wireless connection to that same router.  The router is
providing DHCP Server functionality and serving addresses
192.168.0.100-xxx. All that seems to be working fine.

The ethernet adapter in the desktop showed it was operating as
192.168.0.101 with a subnet mask of 255.255.255.0 which I thought
meant it would only accept packets addressed to 192.168.0.something
and broadcast packets on that same net.  I had also believed that the
4 wired ports on the router were switched, not a hub.  Hence I was
quite surprised that the firewall had seen and denied access to a
request from the laptop 192.168.0.102 (UDP port 49xxx) to
239.255.255.250 port 3702.

Yes, the laptop is a Vista machine and therefore is using their new
discovery protocol, but my question is why the desktop software ever
even saw this message. I know I am missing something very basic
because I would have thought the switch would not even have put the
message on the desktop's wire and that even if it did, the ethernet
port hardware or driver on the desktop would not have passed the
message in to where the firewall would see it.  What am I missing?

-- 
Man is the only animal that laughs and weeps, for he is the only
animal that is struck with the difference between what things are and
what they ought to be.
  - William Hazlitt