Vista - Guest cannot ping host but everything else

Hi,

For a long time, I have had a 2003 guest running on a 2003 host using
Virtual PC.

The host is at 192.168.10.5 and is also the domain controller.
The guest is 192.168.10.223.

There is a default gateway at 192.168.10.1 which is my router to the
Internet.

In Virtual PC, I used the host's NIC for networking. I was able to
connect to the host as well as the Internet as well as all other
machines on the network. No problems at all.

Now I installed Virtual Server 2005R2 on the host and booted up the
guest image. The guest's virtual NIC is connected to the host's
physical NIC, just like in Virtual PC before.

So now, the guest can ping the Internet, any other machine on the
local network EXCEPT the host.

I read about the loopback but I don't think that's the solution to my
problem. I do not want the guest to be on a different class C network.
But I did try it out and I ran into all kinds of issues. With the
loopback, the guest and host could ping each other, but nothing else.
Plus I would have to reconfigure my firewall, port forwarding (to
access the guest from the Internet via RDP), etc.

So why was this working just fine in Virtual PC, but is causing these
problems in Virtual Server?