Wired cable modem to wireless issue

Question

I have a wired 2 port coda-45 cable modem that has a UI but there are no settings available for config, the ISP stuff all config settings over cable.

I can get internet access connecting LAN port from modem to my computer but I can't seem to get wifi access when connecting some other routers that I have to this cable modem. The only thing I could get to work was a raspberry pi configured as a wifi hotspot with ethernet port connected to the cable modem.

Here's what I've tried: The unconfigurable cable modem gateway is on IP 192.168.100.1, I can't see from it's UI status page what the DHCP range is configured to.

I have configured some other wifi routers I have for test to be on the same subnet with DHCP disabled and have tried setting IP to 192.168.100.2 & 192.168.100.253 as I think the address needs to be set to something outside the main cable modems DHCP range but I can't get internet access when connected to the wifi router.

Maybe this means the DHCP range on the cable modem covers all available IP's on subnet 100, I'm not sure.

I also have Netgear GS108E switch and tried setting up 2 x VLANS, one for the the cable modem and the rest for end devices and wifi router but I can't get internet access here either. For this test I re-enabled DHCP in my wireless router which is on 192.168.0 subnet. I'm not so confident I have configured the VLANS correctly though.

I'm not sure what else to try. In theory should I be able to use my switch to bridge 2x subnets with DHCP enabled on both?

Any pointers here is greatly appreciated!

Answer 1

The Hitron CODA-45 is a DOCSIS 3.1 cable modem. It has two Gigabit Ethernet (1000BASE-T) ports, not for connecting multiple devices, but ONLY for bonding together with BOTH connected to a SINGLE device. This is because since DOCSIS 3.1 supports speeds over 1Gbps, so a single 1Gbps Ethernet port doesn't cut it. The hardware to support bonding two 1Gbps ports is sometimes cheaper than the hardware to support a single 2.5Gbps or 10Gbps Ethernet port.

The Hitron CODA-45 is only a modem.

It is not a gateway, router, DHCP server, 2-port Ethernet switch, DNS proxy, or anything else.

(The modem didn't serve your PC a DHCP lease; your PC's DHCP request was transparently passed through the modem to your ISP's network, and something on the ISP's network gave you that DHCP lease.)

The DOCSIS standard requires a pure modem like this to latch onto the first MAC address it sees out its Ethernet port after booting, and only talk to that one device until the modem is rebooted. This has been a stupidly frustrating part of the DOCSIS spec since the beginning, and continues to this day.

So, take any home gateway router with an Ethernet WAN port, configure the router to get its WAN port address via DHCP and to provide NAT service, and to use a different private subnet than whatever the DOCSIS network is assigning you (e.g. different from 192.168.100.0/24 in your case).

Once you've got your router configured, plug the router's WAN port into your modem's main Ethernet port, then unplug power from the modem, wait a moment, and then plug it back in.

Don't plug anything into the secondary Ethernet port of the modem; that is only for port-bonding. If you have a router with two 1Gbps Ethernet WAN ports designed to be bonded like this, you can use that instead.

Make sure your router is going to do NAT, not transparent bridging, between its WAN and LAN sides. If the router does transparent bridging, then the next time the cable modem reboots (maybe a power outage), it might see traffic from a LAN device before it sees traffic from the router itself, and might accidentally latch onto the wrong device. NAT prevents this problem, because it makes all traffic coming out the router's WAN port use the router's WAN port MAC address as the source MAC address.