When i installed gentoo i had to use a USB network device. this took the place of eth0

Once i got my onboard card to work it was given the name eth1

To move these card names around…

•  In your fav editor open (as root)
• /etc/udev/rules.d/70-persistent-net.rules

you will see something like this…
# USB device 0x0707:0x0201 (pegasus)
#SUBSYSTEM==”net”, DRIVERS==”?*”, ATTRS{address}==”00:04:e2:3d:54:b5″, NAME=”eth1″

# PCI device 0x1969:0x1048 (atl1)
SUBSYSTEM==”net”, DRIVERS==”?*”, ATTRS{address}==”00:1d:60:16:22:55″, NAME=”eth0″

• This shows two cards with the first commented out and their names on the end
• Make your changes and restart your machine