OK, I just looked at the trace of 3808 doing a FW update check.
The IP stack actually starts out with a TTL (time to live) of 30. So here's how you can tell if your Denon update will work or not.
If you traceroute to 202.218.213.57 (the address in the packet trace) and you are more than 30 hops away, you will not be able to perform the update.
One more thing, they use a tcp window size of 8192. That's pretty small. Granted it's an AVR but still! The couldn't devote 16K to TCP so the update can happen twice as fast?
UPdate: I just tried to perform a traceroute. Some idiot ISP decided to block ICMP messages so you won't get that far. Oh well. But it does get you close enough to see how much wiggle room you have. When I did the traceroute, I got up to 23 hops. Since my update works, the denon's server's IP can't be more the 6 hops away from the the idiot ISP blocking the ICMPs.
