With virtualization, e.g. with KVM or Xen or lguest, one often wants one's guest machines to appear on the same physical network as the host machine. This is especially true on server machines which have a fixed network connection.

You can achieve this by creating a bridge interface, running dhcp on that and then adding the physical ethernet device to the bridge. From then on, backend network interfaces from guest machines (e.g. tap devices) can be added to the bridge allowing them to appear on the lan.

See this page on the libvirt wiki for how we recommend people configure such a bridge:

  http://wiki.libvirt.org/page/Networking

You'll note that the first step is to disable NetworkManager - that's what we want to fix :-)

We use the name "shared physical interface" to more accurately easily describe this bridge - the fact that we're configuring a bridge to the user is not important, but rather the fact that the physical interface is being configured so that it can be shared with virtual machines. See the "User Visible Concepts" part in this now obsolete write-up:

  http://www.gnome.org/~markmc/virtual-networking.html