to all who read this message, I can't assign the second MAC to the second NIC for a VM. If only use 1 NIC, assigning MAC works. But not for 2 NICs or more. Do anyone has any ideas? I have installed KVM on Ubuntu 11.10 64bit on my laptop. Also created the bridge on the host, and also taps. I created a RHEL 6.1 64bit VM on this environment and run: sudo kvm -m 512 -drive file=/data/vm/kvm/hslt/rhel61_x64_1st,cache=writeback -boot order=c -localtime -net nic,model=virtio,vlan=1,macaddr=52:54:00:12:34:56 -net tap,vlan=1,ifname=tap2,script=no -net nic,model=virtio,vlan=1,macaddr=52:54:00:12:34:55 -net tap,vlan=1,ifname=tap3,script=no It doesn't work and the VM can NOT start. If I change the command to: sudo kvm -m 512 -drive file=/data/vm/kvm/hslt/rhel61_x64_1st,cache=writeback -boot order=c -localtime -net nic,model=virtio,vlan=1,macaddr=52:54:00:12:34:56 -net tap,vlan=1,ifname=tap2,script=no -net nic,model=virtio,vlan=1 -net tap,vlan=1,ifname=tap3,script=no The VM can start, but the second NIC doesn't work in guest VM, and showing like no such device. After change it to: sudo kvm -m 512 -drive file=/data/vm/kvm/hslt/rhel61_x64_1st,cache=writeback -boot order=c -localtime -net nic,model=virtio,vlan=1 -net tap,vlan=1,ifname=tap2,script=no -net nic,model=virtio,vlan=1 -net tap,vlan=1,ifname=tap3,script=no It works well, with 2 NICs in guest VM.