I've been struggling with this issue for 2-3 days trying to get a solution. My router can't get a valid IP address from Comcast so no WiFi - which is a huge problem. I just upgraded to a Gigabit internet plan and upgraded my modem to Arris SB8200 - an approved modem. I've been on the phone with Comcast support and talked to 3 different support techs - all were no help at all, just blaming the modem or the router. The router was working fine with my previous slower service, and the modem is connecting to the internet and works when directly connected to my computer via ethernet cable. I just can't get the router to work.
I've figured out the problem is because I'm trying to use both ethernet output ports on the modem - one to my computer for maximum speed, the other to my router. Apparently (which no one at Comcast seems to know or understand) this will only work if the ISP can provide 2 IP addresses.
My other option is to connect the modem to the router, then connect an ethernet output on the router to my computer. But that cuts my wired download speed from 800+mbs to about 350mbs. Yikes! That's a BIG hit.
I give up with Comcast phone support on this issue - it's just a waste of time getting nowhere. I've seen that several other people have been able to get help on this same issue through the forum so I'm trying that out of desperation! Thanks for anything you can do.
Check your local service area first if Additional IP addresses are available to you. You can look at the rates at https://xfinity.com/support/rate-card (requires account logon)
The entry will look like this or similar (typically on the last page):
If the entry for an additional IP doesn't appear, it's not available in your area.
If it does, let us know, and I'll have Comcast Support take it from here.
Oh, bummer! It doesn't appear to be available in my service area. Can't find it on the rate card. So it looks like my only option is to use only one output ethernet port on my modem, connect to the router, then connect ethernet from router output to computer. Do you think that should work since I can only get 1 IP address?
I thought I tried that before and I ended up with NO internet to anything. The router had the right IP address, but the modem didn't. I just swapped the problem. I'll try it and post my results here.
P.S. I tried to add a screenshot of my rate card like 3 times, but it won't work! It displays when editing the post, but not after saving...
I made the change I suggested in the previous post and IT WORKS! Internet speed to my computer dropped about 200mbs so that's a negative. I'm using an older CAT5e cable. Just ordered a new CAT 7 ethernet cable to replace it. We'll see if that helps any. At least I have WiFi now with good speed throughout the house.