Subnet issue

[Bqtchef]

I have a very busy network so this issue causes a lot of problems for me, I'm hoping you'll all be able to help me.

I have a Motorola MB8611 cable modem after activation (Xfinity) the modem/comcast changes my subnet mask to my pc  255.255.254.0 from the default 255.255.255.0.

I've tried two different scenario with the same results

Scenario 1;  MB8611 to PC with no other network, I'm online  everything is fine but  my subnet is changed to 255.255.254.0  I switch cables around  to mb8611 to router (RT-AX86U) to PC not able to connect due to subnet changed. other pc within the netword has no internet access

I deactivated the MB8611 and reactivate my C7100 netgear cable modem,  I have no issues with this Modem

Scenario 2;  MB8611 to  AX86U to PC with the same results my subnet mask is changed to 255.255.254.0

I deactivated the MB8611 and reactivate my C7100 netgear cable modem, everything is back to normal.

Xfinity has no solutions, called several techs for each scenario. I've search the web found how to's on changing the subnet but the details are lacking for a beginner like me. Anybody here have a solution for me?

There nothing in the MB8611 Modem for me to change even in advance mode, so I'm hoping I can configure the router or my pc

I'm probably leaving out information that I'm not aware of, so please ask.

Thank you

[jzchen]

EDIT:  Are you connecting both modems to the Coax cable with a splitter?

Scenario 2:  MB8611 to AX86U to PC.  Both AX86U and PC are assigned subnet mask of 255.255.254.0?  Is the PC able to access the internet?  If yes just reboot all other devices on the network, and they should all be assigned 255.255.254.0.  If not manually reassign the remainder to 255.255.254.0.

[Bqtchef]

Only the PC is assigned the subNet  mask 255.255.254.0  router at 255.255.255.0  not able to log into router 

[jzchen]

So if router has a subnet of 255.255.255.0 and you reboot the PC what is its subnet mask at that point?

[jzchen]

You’re probably having issues because Netgear uses IP subnet 192.168.0.xxx and ASUS uses 192.168.50.yyy.  Once you have MB8611 connected properly to AX86U, power off all clients then reboot them.  They should all get IPs of 192.168.50.yyy.  Hopefully subnet mask reverts to 255.255.255.0.