Starting with Fedora 7 if you use Xen you will notice a new subnet and a
bridge automagically being created by libvirt. This is great for desktop experimenting, but if you are running this on a server you probably have an opinion on how your network should be configured.
Here's how to revert to manually controlled network settings.
Edit /etc/xen/xend-config.sxp and change "(network-script network-bridge)" to "(network-script /bin/true)".
# cat ifcfg-br0
Now your primary IP of your dom0 lives on the br0 interface. Your domU's will be on the same VLAN and should see your DHCP server. Don't forget to enable IP forwarding.
Allow forwarding across the bridge by modifying iptables.
iptables -I FORWARD -m physdev --physdev-is-bridged -j ACCEPT
service iptables save
You may need this depending on subnetting
sysctl -w net.ipv4.ip_forward=1
Add it to /etc/sysctl.conf to make it stick
# Controls IP packet forwarding
net.ipv4.ip_forward = 1