Ethernet has a self-assigned IP Address & will not be able to connect to the Internet

[Bob5545]

NETGEAR Nighthawk Multi-Gig Speed Cable Modem DOCSIS 3.1 for XFINITY by Comcast

Two (2) Gigabit Ethernet port with auto-sensing technology.

How do I set up this modenm so I can have one ethernet port for my router, and the other ethernet port for my imac.

The problem is with the router connected when I connect an ethernet cable from the 2nd ethernet port, I get that Mac message  "Ethernet has a self-assigned IP Address & will not be able to connect to the Internet".  

The only way I can get that ethernet port to connect to the mac is to restart the modem, and that messes up the connection to the router.  I lose all the wireless internet on the router.

How do I make this work?  I want both ethernet modem ports to work at the same time.  Modem specs say that is how it should work.

Comments.

[FURRYe38]

The CM modem does not have a built in router. The two ports in back are for WAN side IP addresses given to the modem by an ISP. Some ISP do not give out more than 1 IP address to a given modem, even ones with more then 1 WAN port on the back. You can ask your ISP if there is an option to get another WAN IP address on the other port on the modem. If they don't support that, then your limited by just the 1 port and 1 IP address from the ISP. 

[Bob5545]

Two (2) Gigabit Ethernet port with auto-sensing technology.

This modem has auto-sensing technology.

What does that mean?  I thought that meant for each port it would sense the connection and supply the neccessary connection requirments.

What am I missing?

[plemans]

what your missing is the xfinity only assigns 1 public ip address per modem unless you pay for a 2nd ip address or have a business grade account. Meaning you can only connect 1 device to the modem.

Its a limitation because of how xfinity does business.

there's other isp's that assign multple ip addresses per modem but xfinity is only 1 unless you pay for more. 

[Bob5545]

So why did Comcast recommend this modem?  I could have purchased the 1 port ethernet modem?

Not a big deal but sort of messed up my mesh network when I rebooted the modem to hardwire ethernet to my iMac.

[FURRYe38]

Auto sensing is that when a lan cable connected to the modem is plugged in, the port will adjust the connection rate to what the connection rate is of the connected device, i.e. external routers or wired PC/laptops. Most devices nowdays are 1000Mpbs connection rate supporting, however the modem support 100 and I believe down to 10Mpbs for backwards compatibility. 

[FURRYe38]

The CM series of modems is good. Also the CM series support Link Aggregation as well. So if you have a external router that supports WAG on it's WAN side you have some flexibility of using both ports on the CM modem connected to 1 router with it's WAG enabled on it. 

Nothing wrong in having additional ports. 

Just have to remember that for your ISP, unless you ask and pay for the additional IP address, 1 port can only be used at a time on the CM modem. 

[Bob5545]

Thanks for responding.  I have the new Linksys MR9600 Mess Router.  I assume it has all features and functions. 

[plemans]

Assuming is a poor choice. 

That Linksys model doesn't have wan port aggregation that I could find. 

But maybe your next model might. 

[FURRYe38]

Ya, I don't see any LAG support on that model router. 

https://downloads.linksys.com/support/assets/userguide/MR9600_USERGUIDE_INTL_C00.pdf