This depends a bit on what the Linux box is doing for you.  Regardless,
the first two steps are:

1) describe the ethernet card in plan9.ini.  man plan9.ini for details
2) reboot and make sure the card was recognized.  You'll see a line in
the startup noise with the ethernet card type and address

The rest depends on your configuration.

3a)If your plan9 machine is connected directly to the cable modem
or if the Linux system is being a media bridge for it:

	ip/ipconfig -h hostid

should do it.  host id is the host id given to you by your @home
company.  It is used by their dhcp server to distinguish your system,
at least that's what they do where I live.  Dhcp should get the
rest of the info you need like gateway and dns servers to use.
If this works, change your /rc/bin/termrc to reflect it.

3b)If you are going to use your Linux box to actually be an IP
gateway, then either it will have to run a dhcp server or you'll
have to configure everything by hand.  Be sure to read rsc's
/sys/doc/start.ps.  Then you'll end up doing something like:

	ip/ipconfig -g gateway-ip-address ether /net/ether0 your-ip-address you-ip-mask
	echo '	dns=your-dns-server' >> /net/ndb

Cheers.