[CentOS-virt] Second VM cannot use the network.

Hi, I started to use libvirt and kvm yesterday. The first VM works great. Today I made another VM, same configuration as before. But its network just did not work. The guest cannot even get a dhcp ip. The network is NAT mode. The first VM got internal ip correctly. The command I made the VM is: virt-install -nzabbix1 -r2048 --vcpus=2 -c/var/lib/libvirt/images/CentOS-6.0-x86_64-netinstall.iso --os-type=linux --os-variant=rhel6 --disk vol=kvmguests/zabbix1 --vnc -v --virt-type=kvm --check-cpu --prompt --arch=x86_64 --vnclisten=0.0.0.0 --noautoconsole --noreboot --autostart

What should I do?