Initial cable modem setup mystery

[Robulin]

WARNING:  For most ISP home accounts, the ISP will provide only 1 IP address via DHCP.  So, if you follow Netgear's instructions and connect a computer to the modem directly at first, then either add your downstream router on a second port or disconnect the computer and connect the router to the same modem port, the router won't get assigned its IP data for SOME TIME.  It's "stuck" expecting the computer's MAC address for a while.   So, either be patient and wait, or don't bother connecting a computer directly to the modem in the first place.  The modem can be accessed via 192.168.100.1 from your LAN anyway.

And yes, the CM1200 has 4 ethernet ports, but since the ISP will provide only one IP address, and the CM1200 is not a 4-port router, it doesn't route between its ports, it's pointless to connect more than one ethernet device at a time to the modem.

Netgear's instructions should mention that. 

If only Netgear's user interface for the modem showed some information about the ethernet ports, the problem would be easier to diagnose when you have 2 ethernet devices attached or you've recently swapped devices in the same port.

[Robulin · Accepted solution]

It was the delay that had me stumped, the fact that the IP info assigned to the laptop "stuck" for a while.  If I'd just waited, probably 10-15 minutes after swapping the laptop for the router, it would have worked the first time.  Just guessing, almost everyone who buys this cable modem has one-address home ISP service, so the extra 3 ports seem more like marketing than anything else.  If someone's home equipment is sophisticated enough to trunk/aggregate two 1Gbps ports, the ISP is going to have to figure out how to offer speed to match on its coax cable.   Does that happen?  I see Comcast advertises up to 1200Mbps, so maybe it does.  Unless it really provides that speed, the port aggregation seems like a feature that's misplaced in a common home network served by coax cable. 

I think a better set of instructions would be to plug the router into the modem first thing.  The modem can then be accessed at 192.168.100.1 from anywhere in the home LAN.  No delay, no mystery, no problem.

[microchip8]

If the "modem" has more than one ethernet ports, it's a gateway (modem&router). In other words, if you connect another router to it to one of its ethernet ports, and you don't take care of it, you'll be in a double NAT situation which is not recommended!

The ISP providing 1 IP address or more doesn't really matter. Also, LAN traffic is switched, not routed.

[FURRYe38]

FYI, the CM1200 is a MODEM only. No built in router here. 

The 4 ports you see in the back are ISP assigned WAN IP address ports. Thus meaning that if a ISP supports having more then one public WAN IP address to the modem, each WAN port on the CM1200 could have up to 4 externally connected wifi routers. Most ISPs only support having 1 public WAN IP address per house hold. 

So you need a wifi router connected to one of the WAN ports so that you can have more then one client device connected getting services thru the modem. 

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@Robulin wrote:

WARNING:  For most ISP home accounts, the ISP will provide only 1 IP address via DHCP.  So, if you follow Netgear's instructions and connect a computer to the modem directly at first, then either add your downstream router on a second port or disconnect the computer and connect the router to the same modem port, the router won't get assigned its IP data for SOME TIME.  It's "stuck" expecting the computer's MAC address for a while.   So, either be patient and wait, or don't bother connecting a computer directly to the modem in the first place.  The modem can be accessed via 192.168.100.1 from your LAN anyway.

And yes, the CM1200 has 4 ethernet ports, but since the ISP will provide only one IP address, and the CM1200 is not a 4-port router, it doesn't route between its ports, it's pointless to connect more than one ethernet device at a time to the modem.

Netgear's instructions should mention that. 

If only Netgear's user interface for the modem showed some information about the ethernet ports, the problem would be easier to diagnose when you have 2 ethernet devices attached or you've recently swapped devices in the same port.

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[Kitsap]

The CM1200 also supports link aggregation (LAG) if you have a router that supports LAG input.

[Robulin · Accepted solution]

It was the delay that had me stumped, the fact that the IP info assigned to the laptop "stuck" for a while.  If I'd just waited, probably 10-15 minutes after swapping the laptop for the router, it would have worked the first time.  Just guessing, almost everyone who buys this cable modem has one-address home ISP service, so the extra 3 ports seem more like marketing than anything else.  If someone's home equipment is sophisticated enough to trunk/aggregate two 1Gbps ports, the ISP is going to have to figure out how to offer speed to match on its coax cable.   Does that happen?  I see Comcast advertises up to 1200Mbps, so maybe it does.  Unless it really provides that speed, the port aggregation seems like a feature that's misplaced in a common home network served by coax cable. 

I think a better set of instructions would be to plug the router into the modem first thing.  The modem can then be accessed at 192.168.100.1 from anywhere in the home LAN.  No delay, no mystery, no problem.

[FURRYe38]

Since the CM1200 only has 1Gb WAN ports, would not be really supporting of 1.2Gb on a single WAN port. If LAG is enabled, and you have support for LAG on a router or PC, then maybe, otherwise, 1Gb is about max on a signal line and single WAN port.