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      virbr0    Link encap:Ethernet  HWaddr a2:17:ea:e3:47:7e  
      inet addr:192.168.122.1  Bcast:192.168.122.255  Mask:255.255.255.0
      UP BROADCAST MULTICAST  MTU:1500  Metric:1
      RX packets:0 errors:0 dropped:0 overruns:0 frame:0
      TX packets:0 errors:0 dropped:0 overruns:0 carrier:0
      collisions:0 txqueuelen:0 
      RX bytes:0 (0.0 B)  TX bytes:0 (0.0 B)

can anyone tell me what this interface does and how I can change the IP address? I want to see if it is the reason my guests cannot connect.

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2 Answers

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.

It was likely bundles with a VM software you installed at some point. If you'd like to remove it, and you're sure nothing else depends on it, you can use the following command:

sudo brctl delbr virbr0

(from UbuntuForums)

It is unlikely that the virtual bridge is affecting your guests' ability to connect to the Internet, though.

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thanks for the quick response. Do you know how i can change the address that was assigned by default? –  user125242 Jan 22 '13 at 22:11
    
it looks like my guests are being assigned to the virbr0 bridge even though the ip is in a different range and I can not assign the interface to br0. –  user125242 Jan 22 '13 at 22:14
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For changing the ip address of virbr0 under ubuntu 12.04

sudo cp /var/lib/libvirt/network/default.xml /tmp/default.xml
sudo vi /tmp/default.xml # edit the ip address
sudo virsh net-destroy default
sudo virsh net-define /tmp/default.xml
sudo virsh net-start default

You could refer to: http://wiki.libvirt.org/page/Networking

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