eth1 is unknown

[Stephen Thudium]

When I try to bring up Airport Extreme by "ifconfig eth1 up" I get  
the error message "eth1: unknown interface: No such device"

I was following the HOWTO topic "Enabling Airport Extreme"  on an  
iMac and with YDL5.0

Previously I successfully had Airport working (using the HOWTO).

Then it stopped working and I tried repeating the HOWTO with the same  
error.

Lastly, I re-installed YDL5.0 and repeated the HOWTO but got the same  
error.

Any thoughts?