iptables sharing the net

[Donn]

Yo,
On my old Dapper install I used to run this command to allow my laptop to surf 
the net via Kubuntu:
iptables -t nat -I POSTROUTING -s 172.16.200.0/24 -o ppp0 -j MASQUERADE 

(I connect to my ISP via pppoe)

Diagram:
laptop <-----> Kubuntu PC <----> RouterThing <---> net
My Kubuntu PC has 2 eth ports: eth1 is internal (to the laptop) and eth0 is 
external to the router thing (on the roof - wireless net connection).
eth0 : IP is whatever.
eth1 : IP 172.16.200.1

Laptop:
IP 172.16.200.2
Gateway 172.16.200.1

I discovered today that I can ping between the two machines but cannot ping 
form the laptop out to the net -- whether via domain name or IP (I tested 
google).

I saw some message (can't find out where) implying that ipv6 is being used on 
eth1, perhaps that's the problem.

Any suggestions? My network savvy is feeble. :)

\
-- 
I can bend minds with my spoon.
-- unknown