Connect Access point to a second modem.. what would happen?

Question

So my internet had been setup initially with a modem and router (TPLink TL-WR841N) on the second floor of my house. The coverage wasn't great so Verizon came and added a gateway on the ground floor. So i had both the gateway and the modem + Router combo at once. I wanted to consolidate and was able to run an ethernet from the gateway in the basement to the original router. I then setup that original router as an access point. I still have the modem but it is not connected to anything.

My questions, and this is purely for my curiosity as I really don't know too much about the nitty gritty technical details of networking, are:

What would happen if I were to plug that modem into a LAN port on the Access Point? What about if it was plugged into the WAN Port?

Another reason I wanted to create the access point is I was having trouble seeing some devices on my network and it seemed to be dependent on whether they were connected to the router or the gateway.

I was also curious if it is possible to have one cohesive local network with both the gateway, and the modem+Router combo if the two are connected via coaxial cable. There seems to be a splitter on the coax cable that goes into the gateway and I think it may lead to the modem in the other location

Thanks in advance for any insight

Best, Bryant

Answer 1

purely for my curiosity as I really don't know too much about the nitty gritty technical details of networking

You really need to understand the nitty gritty technical details to understand what's going on.

What would happen if I were to plug that modem into a LAN port on the Access Point?

Both the modem and the AP now share a LAN segment. Both the modem and the AP probably run a DHCP server (this is where you need to get into the nitty gritty details), so they both try to assign IP addresses to any device on that segment, while not listening to the DHCP protocols to set their own addresses. So either by chance they happen to use the same segment (192.168.0.*/24 is a popular choice), which means in theory they could communicate, unless they assign themselves the same IP. Or they don't, which means IP routing doesn't match, so they won't communicate.

Other configurations may lead to other scenarios. None of those scenarios does anything useful.

What about if it was plugged into the WAN Port?

Then the AP uses DHCP provided by the modem to set its gateway address, and will route all traffic to this gateway address (at least in the default configuration), so it will end up in a modem not connected to your ISP.

I was also curious if it is possible to have one cohesive local network with both the gateway, and the modem+Router combo if the two are connected via coaxial cable.

No.

There is a way to have one cohesive local network: Configure your TL-WR841N to bridge the WLAN AP and the LAN (and while you are at it, bridge the WAN port to the LAN). Then you can connect an ethernet cable to either WAN or LAN on the 841, connect the other end to the LAN on Verizon gateway, and everything will form one cohesive network.

AFAIK the firmware on the 841 doesn't allow this, so you'll need firmware that does. OpenWRT has become too big for the 841, so e.g. get an old version of OpenWRT. To make all of this work, you need to understand the nitty gritty details so you can set up the 841 correctly.