Modem and router have same IP Address

[DonnyB]

I changed from Spectrum to CenturyLink. The Spectrum-supplied modem had an IP address of 192.168.1.1 and my TP-Link AX1800 router has an IP address of 192.168.0.1
When I switched to CL, they gave me a Greenwave C4000 modem with an IP address of 192.168.0.1
The tech plugged it all in and the C4000 modem and the AX1800 router worked without any problems. To access the C4000 setup the 192.168.0.1 takes me into the setup, no problem. The problem is I do not know how to access the AX1800 since it has(had) the same IP address as the C4000.
The connections are: Wan via DSL POTS line into the C4000. LAN#1 on the C4000 goes to the WAN port on the AX1800. A LAN port on the AX1800 goes into the desktop computer. The desktop has Win10 and does NOT have a wifi radio, only an ethernet port.

 

[dev_null]

Disconnect the router from the modem. Log into the router and change the LAN > DHCP server range (to 192.168.2.1 or something not in the same range), click save, wait for the UI and then change the LAN > LAN-IP IP address and save again (I believe that is the correct order; I haven't done it in a while).

Then reconnect the modem and you should be good to go.

 

[DonnyB]

Thanks for the help. When I unplugged the CL modem, the AX1800 router is smart enough that it automatically changed from 192.168.0.1 to 192.168.1.1.
Not sure I understand how that worked, but it made my job much easier.
BTW, the CL tech had high praise for the AX1800 router. So far, for me, it has been a really solid router.

 

[aex.perez]

Being on AT&T Fiber with their modem I use passthrough mode, giving the wan port of the router the same IP address as modem. Just curious whether one would use the DMZ option ( https://www.centurylink.com/home/help/internet/modems-and-routers/advanced-setup/dmz-hosting.html ) to do the same. Not a user of this brand of router, so don't know how robust it's security is as DMZ mode bypases the modem's FW and is then up to the router. Or if any of your devices are ususing the modems wifi or other lan ports. But if everything is getting internet through the AX1800, and it's security us robust, DNZ mode on the modem could be another way...

 

[Cake]

I would have thought there would be a "bridge mode" on the modem. Maybe that's what the passthrough mode is.

 

[aex.perez]

I would have thought there would be a "bridge mode" on the modem. Maybe that's what the passthrough mode is.

That was the big debate when AT& Fiber rolled out. Bridge mode, the termination/authentication is at your gear. Passthrough (sometimes called DMZplus), the termination/authentication is at the modem allowing the ISP (AT&T) to manage it. Not sure with the DMZ capability, but likely similar to passthrough where the ISP can manage the modem...

or put another way

"The bridge mode and IP passthrough mode both provide similar functionality where entire traffic is pass-through the gateway and the public IP is assigned to the customer's router behind the gateway. The bridge mode does not terminate the traffic at the gateway while the IP passthrough does terminate the traffic at the gateway. In order for ISPs to connect to the gateway, the IP passthrough mode should be used instead of the bridge mode." - https://www.iplocation.net/bridge-vs-passthrough

You still we be abled to get to the modem with its private IP via the network behind the router, at least I can.