SOLVED Fairly knowledgeable user needs help with old wireless router and new cable modem

[Vern11]

Last week, I lost internet from Comcast/xfinity. It was not caused by an outage, and I confirmed my cabling was fine, so the problem must be with my settings and/or equipment (I had been using a Motorola Surfboard cable modem and a TP-LINK Archer C8 Wireless Router). When I contacted Comcast, they did some testing and reported that they were certain the problem was a bad modem, so long story short, I bought a new Motorola MB7621 cable modem. However, after exhausting effort, I still can't get my internet service using both the modem and the router working for more than a few moments at a time, and that only occasionally.

My goal is to get my new modem to work with my older TP-LINK router using the settings which worked perfectly about a week ago.

I don't think this is a hardware problem, at least with the modem, because in order to post this I've connected this Windows 10 Pro desktop directly to the modem. Instead, I think the problem lies with the various address settings: IP, Gateway, and DNS. Before the new modem, I set the Default Gateway address on the TP-LINK router to 192.168.1.100, with all other devices' IPs set starting with 192.168.1.101 and so on. I had been using Open DNS, but until I get everything working again I'll use the traditional Comcast DNS servers at 75.75.75.75 / 75.75.76.76 (unless I should be using other Comcast DNS servers).

So with the exception of the DNS servers, the TP-LINK router has all the same settings it's had for years, and I prefer to keep things that way. SO the help I need is guidance on how to set up my modem and router so those settings will work again, OR adjusted settings that are as close as possible. With the direct connection to the modem that I'm using to post here, here are the current settings (from IpConfig):

IPv4 Address: 68.40.3.47(Preferred)
Subnet Mask: 255.255.248.0
Default Gateway: 192.168.1.100 -AND- 68.40.0.1
DHCP Server: 96.113.150.15 [ WTF? ]
DNS Servers: 75.75.75.75 / 75.75.76.76

So I suppose the IPv4 address above must be the one assigned by Comcast for the modem, but what's the deal with the DHCP Server address? Is that assigned by Comcast, or perhaps by Motorola, or TP-LINK?

The only way I can get the Ethernet internet working -- even if never more than about 2 minutes -- is by using fixed addresses, which disables the DHCP, which seems to be important since that's the only way I can get it to work. When I started typing this OP, I was connected through the modem & router using the old values that worked previously. But since that only lasted so briefly, to continue I've had to connect directly to the modem with the addresses above.

So I really think the weird DHCP server address is involved with this problem, but it might not be the only thing. I'm desperate for your help, network-smart folks! Thanks

 

[Trouble]

but what's the deal with the DHCP Server address? Is that assigned by Comcast, or perhaps by Motorola, or TP-LINK?

All the addresses you've listed are part of a class A subnet (0 to 126) and the DHCP server address falls well within that scope of addresses.
My concern is where the 192.168.1.100 is coming from as a default gateway that's a Class C address and wouldn't work at all as a gateway given the other numbers provided.

However typically I would have expected all the address to be in the 10.xxx.xxx.xxx range as that is what you commonly see from a Comcast / xFinity router on the LAN side unless it has been Bridged.
Something similar to and looking like this....

Motorola MB7621 cable modem

I'm not familar with that particular modem, nor how it behaves.
Does is default to Bridge Mode?
When you plug in the Motorola to the WAN port of your C8 and you look at the config page of the C8, specifically the WAN, what are you seeing.

 

[Tim Locke]

My router/modem supplied by Rogers Canada provide a 10. net for IPv4 plus a /64 prefix for IPV6 and is supposedly almost identical to Comcast Xfinity device.
I do not have mine in bridge mode as I found that its inbuilt router is better than the router I was using!
The Rogers network for these new modems is, I think ipv6 internally as it supports TV, alarm systems and a VOIP home phone as well.



I *suspect* that if the new modem is put into Bridge mode and the port #1 on the modem is plugged into the routers wan port it will all come alive.

 

[Vern11]

All the addresses you've listed are part of a class A subnet (0 to 126) and the DHCP server address falls well within that scope of addresses.
My concern is where the 192.168.1.100 is coming from as a default gateway that's a Class C address and wouldn't work at all as a gateway given the other numbers provided.

However typically I would have expected all the address to be in the 10.xxx.xxx.xxx range as that is what you commonly see from a Comcast / xFinity router on the LAN side unless it has been Bridged.
Something similar to and looking like this....
View attachment 13969


I'm not familar with that particular modem, nor how it behaves.
Does is default to Bridge Mode?
When you plug in the Motorola to the WAN port of your C8 and you look at the config page of the C8, specifically the WAN, what are you seeing.

Oh, most wise Noob Listener, I am incredibly grateful for your assistance! Unfortunately, I am still in need of your time and patience. And clearly I need to downgrade my "Fairly Knowledgeable" status back to "ignorant noob"... Here is a list (in no particular order) of the issues I haven't been able to resolve:

First, at some point after this situation began, I've been unable to login to the web UI on the router, with a direct ethernet connection to the router or through the normal modem->router->computer. It fails using either "http://tplinklogin.net" (as the v1 manual calls for), or using the more recent "http://tplinkwifi.net/webpages/login.html". I tried both using either full auto IP & DNS, and I also tried entering the Windows IP settings so that they match what you show in your post, but neither succeeds.

You ask where the "192.168.1.100" gateway address came from, but even though it has worked fine for more than a decade, it was not at all an informed choice. I just picked it out of my ass, so I'm extremely eager to follow your instructions and advice on proper configuration.

But I just discovered something when I'm connected directly to the modem (as I must do in order to get any internet access at all): When I login to the modem's web UI, it's URL is "http://192.168.100.1/MotoHome.asp". I don't see any way of setting or changing that address, so I don't know were it's coming from, but wouldn't you think that would be very important in correctly configuring my setup?

In my OP, I shortened a sequence of events with the phrase "... so long story short...". What actually happened is that when I lost internet access and Comcast told me I had a defective modem, I couldn't afford a new modem just then, so I decided to rent one from them. Nothing worked correctly at all with that one, so I got them to replace it with an identical unit, an Ariss model something or other, which is a combo modem & wireless router. When I couldn't get that one to work either, I waited until I could afford it and bought the Motorola, which (for whatever it's worth) is labeled as Amazon's Choice buy.

The point is, do you think the TP-LINK router is at fault? Especially since I can't log in to the router? Should I try one of the Comcast units again? Or does the modem's IP of "192.168.100.1" need to be part of the configuration?

I sit in rapt attention, master!

ETA: I've attached a PDF with the Windows networking diagnostic report.

 

[Vern11]

Well, I decided that the most likely source of the problem -- at least after setting up the new Motorola modem -- was my existing TP-Link C8 router, especially since I couldn't log in to it. So I ordered a new TP-Link Archer A9 AC1900 Wireless MU-MIMO Gigabit Router, plugged it in, and was able to successfully log in over Ethernet. Everything appears to be working now!

My only complaint is that, unlike the original C8, you can't enter MAC addresses using cut and paste with this new one. You have to painstakingly enter every nibble manually, one at a time (dammit!) But I suppose that's a small price to pay for working internet.

I want to thank both of the users who so kindly replied!

 

[Tim Locke]

Question. Did you just take the new router out of the box and plug it in to the modem? I.e the first line of the quick setup? Let the router sort out who and what it is talking to on the LAN side.

A listing of Ipconfig /all would be interesting now it is working.