I a server hardware fail today and had to swap the hard drive into another server. When I booted up the server it did not have an ip address as eth0 wasnt found.
This was a simple fix all I did was delete /etc/udev/rules.d/70-persistent-net.rules
I then rebooted the server and it greated a new file with the installed network card configured as eth0. This got the networking working as it should be.