virtualbox bridged networking

[stan]

I am trying to set up a new 10.4 machine to be a virtual host for several
virtualbox instances. I have a working example in 9.04 that was done by a
contractor, who has since left. in /etc/network/interfaces O find the
following:

auto br0
iface br0 inet static
address X.Y.106.161
broadcast X.Y.106.255
netmask 255.255.255.128
gateway X.Y.106.143
bridge_ports eth0
bridge_fd 2.5

And that is all, that is not cometned out. There is a bunch of stiff
commented out, so clearly he had a bit of a strugle getting this to work. 
What I am trying to acomplish is to have the host machien, and the virtual
machines appear to be independent machines to the outsdes world, and this
is working fine on the example machine. The existing machine cane be
conected to at X.Y.106.160, which is clearly not in the interfaces file at
all. 

In /etc/vbox, all I cna find is:a file namned vbox.cfg, whihc contains only

# VirtualBox installation directory
INSTALL_DIR="/usr/lib/virtualbox"

How does the host machine get it's iP address? 

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