Eth0 will get is IP from DHCP (Im assuming your running cable or
dsl), but your internel network card needs to be something like
192.168.x.x where the x = 0-255 , ie. 192.168.0.1 or
192.168.0.254. 192.168.0.0 and 192.168.0.255 are reserved IP
addresses for your network address and broadcast address. A
really good read on subnetting is in the PET section. Also the
basic networking how-to at
http://www.ibiblio.org/pub/Linux/docs/HOWTO/ will provide loads
of info.