cable modem

i have a motorola ARRIS SBG6580 connected to my xfinity cable modem via lan (cable feature not in use) when i configure the devices none shows an ip address which i used example 192.168.10.1.... they show the address coming from the xfinity modem but wifi devices can access the Arris via the default 192.168.10.1......yyyyyy

I'm not sure the SBG6580 can be used that way. It has no upstream Ethernet port, and without custom firmware (against DOCSIS device rules), I doubt you can change the routing table to get an Ethernet port used as upstream instead of the coax.

Why not just use the SBG6580 as the modem, as it is intended to be used?

because to use cable i would have to pay for an additional ip address from xfinity so i ran a lan cable from the xfinity cablr modem to the 1st lan port of the SBG6580. the lan and wifi works tho but what i dont understand is the ip address part

You can't really use it that way. You have to use an input port (in this case via coaxial cabling) to output an IP address to your other devices.

Ports 1-4 on that modem are just outgoing Ethernet ports, you can't use them to output an IP address to devices on the other ports because it wasn't designed to do that.

Without an input plugged in it would just use the device's default IP table.

The only way this might work is if you plug your LAN/Ethernet cable into an Ethernet Switch, plug your other devices into that, and take the modem out of the loop completely.

ok well i will use it as is because i use the wifi feature as well