XEN - (magic) networking issue

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http://serverfault.com – I have a very strange situation and do not know where to look. I have a XEN installed on Ubuntu 12.04 LTS. My hosting-provider does require, that every new IP my server uses has an own MAC-address. Therefore I use a bridged interface xenbr0. Now the VERY strange thing: I assign the mac-address to a virtual interface of a virtual machine with Windows 7, it does say "no network connection" - I used dhcp, then entered gateway,netmask&ip manually, flushed ARP cache. Nothing. It still has no connection. I can not ping the gateway. I assign the same mac-address to a virtual interface (the vir (HowTos)