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

[RLRL]

iMac El Capitan

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 modem 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 iMac 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.

And then when I disconnect the Ethernet connection, I have to restart the router to establish wireless internet.



How do I make this work? I want both Ethernet modem ports to work at the same time without turning off the Modem. Modem specs say that is how it should work.



Comments.

 

[barbu]

I wouldn’t recommend connecting your Mac directly to the modem. Just connect to your router. Is there a specific reason for this arrangement?

 

[CWallace]

It looks like that modem only has one IP address to hand out and either the router or the iMac is taking it (depending on which device gets it first), leaving nothing for the other. So if the router gets it, the iMac has nothing and if the iMac gets it, the router has nothing.

So you need to connect the iMac to the router as the router should be able to server as a DHCP server and assign a routable IP to the iMac to allow it to access the Internet. The other Ethernet port on the modem should remain unused.

 

[wilberforce]

I believe the multiple ethernet ports are for the purposes of ethernet link aggregation (multiple physical connections, but a single logical connection) to a single router (that is compatible with link aggregation), not for the purposes of connecting multiple devices. Basically you get higher speed by having two parallel "pipes" connecting your modem to your router.
User's manual should be able to explain.

 

[PinkyMacGodess]

You could also put a switch between the modem and the wifi, and connect the wifi to it, and a computer.

I have always, working form the outside in, had the modem, a firewall on the edge, and a switch, and then put my wifi off the switch. Works every time. (so far) I have had to set the switch MAC address in the past, but that was decades ago.