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Cable Company Home Network Configuration Question

I'd be grateful for any help that will enable me to understand what I can't seem to grasp about my home network's configuration which was set up by Time Warner.

The network cabinet contains two devices - an ARRIS DG860 cable modem/router and an ARRIS TG862G Wi-fi router. Neither is directly connected to the other. Both have coax connections directly to the outside world.

Only the TG862G shows in any Network Device map on my PC by name. It's IP address is 192.168.0.1. It provides the wireless access to which all of our wi-fi enabled devices connect. It has four Ethernet ports on the back, none of which are occupied.

Although the D860 cable modem doesn't appear on the Windows network map by name, I presume it occupies the place in the diagram designated by either "Switch", "Gateway" or both. The D860 also has four Ethernet ports into which all of the wired trunk lines to the patch panel feed. From there, I've used gigabit switches and done my best to spread the data load across the various trunks to avoid bottlenecks.

Everything works fine, but I'm having trouble understanding something. The D860 obviously is acting as more than a switch. It's also performing router duties because it's assigning IP addresses to each device that ultimately connects to one of its four Ethernet ports.

That leaves me questioning how the wi-fi router is configured and the purpose it serves beyond an access point. Hoping it was acting as merely an access point and switch, I tried connecting a home automation system's gateway module into one of the TG862's Ethernet ports for convenience's sake with no luck whatsoever. If it were acting as merely a switch, it should simply plug and play without problem.

Since both the modem and wi-fi router can't be assigning IP addresses without conflict, only one must be doing the job. What I can't reconcile is how the trunk lines to the patch panel and all of the network devices connected to them are receiving IP addresses without ever having anything to do with the wi-fi router, meaning the cable modem/router must be performing that function. Yet wi-fi devices are connecting to the TG862G and being assigned IP addresses without ever connecting directly to the cable modem switch/gateway.

Can anybody help me understand the interrelation and hierarchy? Is the DG860 actually performing all IP assignment and routing which the wi-fi router is simply passing along, or is the cable modem taking its cues from the wi-fi router? If so, why do the wi-fi router's Ethernet ports seem to be in conflict? Is there some kind of transparent handshaking between the two devices that coordinates everything? Since the DG860 doesn't seem to be IP address-accessible, it would seem to be doing the job of a dumb switch, but the trunk lines feeding into and out of it tell a different tale.

I'm certain this is a pre-Networking 101 question that will cause most to scoff at my relative ignorance, but I'd be grateful to anybody who may be able to help me better understand what's going on.

Many thanks in advance.

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