After the kvm virtualization environment is installed, ifconfig will find an additional virtual network card virbr0.
This is generated after the libvirt service is installed and enabled, libvirt generates a
virtual network switch (virbr0), all virtual machines (guests) on the host pass through this virbr0
connect. By default virbr0 uses NAT mode (using IP Masquerade),
So in this case, the guest can access the outside only through the host.
(1)brctl show
(2)virsh net-list
(3)virsh net-destroy default
Network default destroyed
(4)virsh net-undefine default
(5)service libvirtd restart
redhat7:systemctl restart libvirtd.service
Re-use the command: virsh net-list and brctl show to check, and found that there is no more.