I doubt you need to open any ports to get the outgoing connectivity to work. Just as you don't need to open any ports for a computer to access the internet. If you were wanting to access your TiVo remotely then thats another story.
What does it say under network diagnostics on the TiVoHD? Is that where you saw the 192.168.1.6 address or did you look in the DHCP table on your router?
Supposing you did have to open ports, you would normally put the IP of the TiVo as the 'internal' address, with the same port number on the internal and external sides. But as I said there is no need to create 'pin hole' 'virtual server' or 'reverse NAT' entries on your router.
If it has a very strict (outgoing traffic) firewall built in, then you might need to allow certain ports to all or to specific IP addresses (unlikely).

Please also confirm 168.192.1.6 was a typo (rather than 192.168...) asd this is not a valid LAN address.