For the impatient, here is our simple script. We’ll explain it afterwards. This is assuming that you’re on a 192.168.1.0/24 network with no DHCP server.

#!/bin/sh

PATH=$PATH:/usr/sbin:/sbin

sudo brctl addbr br0

sudo ifconfig eth0 0.0.0.0

sudo brctl addif br0 eth0

sudo ifconfig br0 192.168.1.120 netmask 255.255.255.0 up

sudo route add -net 192.168.1.0 netmask 255.255.255.0 br0

sudo route add default gw 192.168.1.1 br0

sudo tunctl -b -u john

sudo ifconfig tap0 up

sudo brctl addif br0 tap0

export SDL_VIDEO_X11_DGAMOUSE=0

sudo iptables -I RH-Firewall-1-INPUT -i br0 -j ACCEPT

qemu-kvm ~/win2k.img -m 512 -net nic -net tap,ifname=tap0,script=no

via HowTos/KVM – CentOS Wiki.