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I just solved my port forwarding problem on the Arris TG1682G. I've been pulling out my hair!
I set up the static IP address in the Arris for my end nodes on my internal home network, then entered the forwarding information in the forwarding table on the Arris. I checked that my Arris indeed had the right ports open by using the web site canyouseeme.org (a nice site to use!) that the ports were properly opened by the Arris. They were. That's what you want to confirm as a first check.
Next, I wanted to see if my internal device would actually respond properly when i tried to log into it using the "router address plus forwardiing port." This was my last check that all was right again after changing routers.
But when I tried to use my laptop and log into my forwarded internal device using the "Arris gateway IP address plus the internal port number" as an IP address submitted from my laptop, to check that my forwarded device would answer, I got nothing but a time-out message as an error. This method of checking has worked fine on my older routers, and the last one I had was also an Arris unit, but older.
What I later found, after much lost time, was that this particular Arrris model that I now have, apparently, does not let you log into your local forwarded devlce by using the external IP address. I've never had this happen on other routers. No one at Comcast ever mentioned this little difference in the several trouble calls I had with them, so i wasted lots of my time before I fugured it out on my own.
So in case your are forwarding ports on this model Arris, you might try checking the final result of your efforts by using your smart phone or other device that is on a different network from your home network and see if the forwarding is working properly. That's the only way this particular Arris model will forward to the desired internal device.
Again, all the previous routers I've had were "smart enough" to do the proper NAT if I used the external IP address on the internal network. Not this one!
Hope this helps someone not waste their time like I have. And, thanks again Comcast for not helping me solve this issue.
Now I have a term, "NAT Loopback" to go with my issue. I learned the hard way that this feature does not exist in some routers -- particularly my new one. Thanks for the bit of terminology.