Hi,
Thought I'd share this fix that took me a while to figure out:
If your using BT Infinity, which should be configured on the EWAN port as PPPoE, you might have issues trying to ping anything on your home network (any host connected to the 7800N).
Anything other than localhost will resolve to 92.242.132.15 and time out accordingly.
To fix you need to opt out of the BT Web Address help service here:
http://preferences.webaddresshelp.bt.co ... rences.cgi
Then restart the router.
Looks like BT are messing with root hints in some way, so the BT dns servers tryto resolve your internal addresses.