Cable modem will only connect to one device through a router

DominionSeraph

Diamond Member
Jul 22, 2009
8,391
31
91
I bought a Netgear CM500V cable modem hoping to free myself from the cable rental fee, but can't get it to work with any of the routers I have. The internet works great when plugged directly into the cable modem, but when hooked up to a router I can get only get internet through it to one device. If the WiFi grabs the connection then all the LAN ports are dead and they show up as either Not Connected or Unknown Connection (No internet) on the attached computer in Windows, and the router's setup shows no connected wired devices. If a LAN port snags the connection it then the WiFi has no internet and the rest of the LAN ports are still dead.

It does the same thing on all three routers -- a WNDR3400v3, WPN824N, and WNR2000v3, which all work with the Xfinity modem/router. (double NAT),
I sent the cable modem back and got a replacement and it does the same thing.

Any ideas? I mean, this is something that should just work, right?
 
Last edited:

Ichinisan

Lifer
Oct 9, 2002
28,298
1,234
136
[edit]
You might be encountering a conflict where the router has the same internal address and DHCP range as the modem.
 
Last edited:

DominionSeraph

Diamond Member
Jul 22, 2009
8,391
31
91
[edit]
You might be encountering a conflict where the router has the same internal address and DHCP range as the modem.

So you think Netgear may have screwed up and set things so their modems don't work with their routers?
Is there a way to change these settings? I'm going to need a new router if I keep this modem since I need something with gigabit, but I'd like to confirm that it will actually work before I put more money into this.

It looks like the cable modem is 192.168.100.1 and the router is 192.168.1.1
 

ch33zw1z

Lifer
Nov 4, 2004
37,759
18,038
146
You're plugging modem into wan port on router?

When you plug into modem directly and i s working, what's ipconfig say?
 
  • Like
Reactions: dave_the_nerd

VirtualLarry

No Lifer
Aug 25, 2001
56,299
10,021
126
First off, is this really a cable MODEM that you have, or a cable GATEWAY?

If it's truly just a cable modem, then you NEED to plug it into the port labeled "WAN" or "Internet" on your router, you can't just plug it into any old wired port.

It sounds like from your description, that this is your problem, OR you have a cable Gateway, and don't realize it.

Edit: You DID go through the "Xfinity walled garden" setup, right? You know that you can't just plug in a new modem and go, right? You need to register your equipment (modem) MAC address with Xfinity.
 
Last edited:

mxnerd

Diamond Member
Jul 6, 2007
6,799
1,101
126
You need to call ISP Comcast that you have a new modem and give them the MAC address of the modem.
 
  • Like
Reactions: ch33zw1z

VirtualLarry

No Lifer
Aug 25, 2001
56,299
10,021
126
Oh, and in case this is somehow a problem with your router settings (with all three routers? I doubt it, but anyways...), make sure that NAT is enabled, and that your DHCP pool settings are large enough to accommodate all of your PCs.
 
Feb 25, 2011
16,786
1,468
126
So you think Netgear may have screwed up and set things so their modems don't work with their routers?

No he's saying the modem is a router.

It wouldn't need to conflict - double Nat causes all sorts of weird issues.
Is there a way to change these settings? I'm going to need a new router if I keep this modem since I need something with gigabit, but I'd like to confirm that it will actually work before I put more money into this.

It looks like the cable modem is 192.168.100.1 and the router is 192.168.1.1

What external IP is the router getting? What external IP is the computer getting? Full output if "ipconfig /all" please.
 

VirtualLarry

No Lifer
Aug 25, 2001
56,299
10,021
126
No he's saying the modem is a router.
I wonder what's really going on here. I looked up the "modem", that he specified, and it's a Cable Modem / Telephony Gateway. But not an "Internet Gateway".

So, I wonder what kind of shenanigans that Netgear might be pulling, to put the VoIP support in, without making the unit a visible Gateway.

I mean, maybe there's nothing amiss at all here, and I'm reading too much into it. But I'm imagining some sort of router-come-transparent-bridge-with-voice-services somehow.

Edit: I still think that OP simply plugged it in, and forgot to do the Walled Garden procedure with Xfinity.
 
Feb 25, 2011
16,786
1,468
126
I wonder what's really going on here. I looked up the "modem", that he specified, and it's a Cable Modem / Telephony Gateway. But not an "Internet Gateway".

So, I wonder what kind of shenanigans that Netgear might be pulling, to put the VoIP support in, without making the unit a visible Gateway.

I mean, maybe there's nothing amiss at all here, and I'm reading too much into it. But I'm imagining some sort of router-come-transparent-bridge-with-voice-services somehow.

Edit: I still think that OP simply plugged it in, and forgot to do the Walled Garden procedure with Xfinity.

I'd be curious what the external/WAN IP of the second router is. That would be more informative.

If you've got modem (w/ router #1) -> WAN port of router #2 -> multiple devices, and only one device can pull an IP and get online at a time, you've probably managed to bung up the DHCP server on router #2 somehow, or plugged router 1 and 2 together on the wrong port.

The more I think about it, the more I think ch33zwhiz is right.
 
Last edited:

JackMDS

Elite Member
Super Moderator
Oct 25, 1999
29,470
385
126
It is a matter of probability.

Even when the Modem is Really just a Pure Modem some time it will not work with 3rd party Modem that is not ""Rented""
from the ISP.

The probability of Not working rises when the Modem is not a pure Modem but a combo of some sort with VOIP/Telephony or Wireless etc.

Why? Because the ISPs have their own concocted settings that Might work with the Modem that they gave, and will not work the same make and model that One got in the free market.

Right or wrong, does not matter it is what it is......


:cool:
 

razel

Platinum Member
May 14, 2002
2,337
90
101
Interesting. I have the CM500 which is the same modem minus the telephone voice. Like most cable modems, the CM500 marries itself to the 1st thing successfully connects to it on power up. Once it's established a connection to one MAC address of the LAN port, it will not recognize another except for very few routers that know how to get it to divorce and remarry. I've noticed, stock firmware from Belkin and onHub/GWiFi appear to do this.

So, power the modem off. Make sure your router is already powered, connect that modem's LAN port to your router's WAN port, then power the modem on.
 

ch33zw1z

Lifer
Nov 4, 2004
37,759
18,038
146
Also, if the modem has a battery to keep the phones alive for a few during power outages, unplug that during power cycle
 

DominionSeraph

Diamond Member
Jul 22, 2009
8,391
31
91
Ok, there seemed to be a combination of issues that was hampering my troubleshooting. Namely my desktop's ethernet port is very finicky and so was showing Not Connected on two of the routers simply because it didn't like them, one of my routers was broken, and I'm not sure the extender I was trying to use is set up for use as a router. (It has no dedicated WAN port)

This thread kept me at it. I had the cable modem all packed up and ready to ship back but decided to do a swap for a fifth time and try to work out what was going on with ipconfig and see if I could piece something together with static IPs. After getting my wired laptop and wifi going at the same time on the WNDR3400 and noticing it was just a port issue on my desktop that was the problem there, I went out and picked up a Linksys EA6300 for $25 and now everything works great.
 

VirtualLarry

No Lifer
Aug 25, 2001
56,299
10,021
126
Ok, there seemed to be a combination of issues that was hampering my troubleshooting. Namely my desktop's ethernet port is very finicky and so was showing Not Connected on two of the routers simply because it didn't like them, one of my routers was broken, and I'm not sure the extender I was trying to use is set up for use as a router. (It has no dedicated WAN port)
I'm very glad that you got it working, but please dude, let us know ahead of time that you are working with flaky / not-known-good kit. It may cause us to widen our solution search-space to consider those possibilities.
 
Last edited:

DominionSeraph

Diamond Member
Jul 22, 2009
8,391
31
91
I didn't know the extent of the issue. My desktop had never required more than a wiggle on the LAN cable to get working, and I recall going through the 3400 at one point. I wasn't expecting it to not work at all on that and the 824N when it was working on the Xfinity router and the extender.
I still have no idea why I was getting Unknown Connection on the laptop, either. I farted around with changing the IP in the router before hooking the cable modem up again so there may have been an IP conflict which that cleared.
 

Charles William

Junior Member
Aug 17, 2018
4
0
1
This may be occurs due to problems in your router settings, make sure each and every connections are proper and enabled, check your DHCP pool settings also.