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I have created a bridge using to existing network interfaces:

$ brctl addif br0 eth1
$ brctl addif br0 eth2

Why do eth1 and eth2 lose their IP addresses? What's the reasoning behind it?

PS this is on Ubuntu Linux.

1 Answer 1

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When a device is part of a bridge the IP address is set on the bridge rather than on the individual devices, because that is where packets can enter from the host system or exit to the host system.

A bridge is basically just a virtual ethernet switch with both the bridge device, and each device that is added to it, being a port on that switch. The bridge device (br0 in this case) is then the switch port which is connected to your computer and which can be given an address and used to send and receive packets.

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  • @TomH Could you please elaborate a bit more? The packets must necessarily come in from the LAN to the bridge first (via eth1 or eth2 only, they being physical devices unlike the virtual br0), right?
    – Harry
    Commented Dec 21, 2016 at 14:52
  • Yes but because the device is part of a bridge it just passes the packets straight to the bridge device and it is the bridge device which introduces them to the IP stack.
    – TomH
    Commented Dec 21, 2016 at 16:41

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