The virbr0, or "Virtual Bridge 0" interface is used for NAT (Network Address Translation). It is provided by the libvirt library, and virtual environments sometimes use it to connect to the outside network.
Type the following command:
$ ifconfig
Sample outputs:
virbr0 Link encap:Ethernet HWaddr 00:00:00:00:00:00
inet addr:192.168.122.1 Bcast:192.168.122.255 Mask:255.255.255.0
inet6 addr: fe80::200:ff:fe00:0/64 Scope:Link
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:0 errors:0 dropped:0 overruns:0 frame:0
TX packets:39 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:0
RX bytes:0 (0.0 b) TX bytes:7921 (7.7 KiB)
Remove that bridge vrbr0 will make it come back.
$ sudo ifconfig virbr0 down
$ sudo brctl delbr virbr0
Now start the 'default' network using virsh command.
$ sudo virsh net-start default
This will automatically re-create the virbr0 bridge.