New to cable modem and am confused

[Odyssey1942]

We bought a home previously owned by a Cisco certified engineer. He had ATT U-Verse as his ISP. His connection came in through his office (which was upstairs). He used an ethernet cable in the wall to get the signal into the office closet where there is, for want of correct terminology, a jumper board, maybe a patch panel?

The board has ethernet wiring attached on the outside of each of two vertically positioned parallel female ethernet jacks, let's number the left most 1-11 (odd), and the right most 2-12. The signal from ATT entered on jack #3. Jack #5 goes to the office in the house. #7 goes to the living/TV room at the other end of the house (more on this later). Can one add a photo to a post? If someone could steer me to the directions, I will upload a pic of the board.

The co-ax where TWC enters is near the old ATT jack in the office so I have connected one end of an ethernet cable to the output of the cable modem and the other to the same ethernet jack that he used to send his ATT signal to the jumper board. So from the jumper board in the closet, all should operate much the same way.

I first connected jacks 3 and 5 by an ethernet cable to get the signal (successfully) to the office.

I now find that if I have my office computer connected to the internet by wifi in my office (which is downstairs), and decide to unplug the access point (to use it elsewhere) and plug my computer in directly by ethernet cable, I have to reset the modem (a Ubee DDM3521). Same if I am connected directly, unplug and then plug in the wifi access point (and the modem must be reset before the access point.)

Is this normal? Please confirm that it is necessary to reboot the modem each time something is plugged or changed around in downstream. Reason for asking is that maybe I have a problem that I am mis-diagnosing.

More questions to follow but the wording may be different as I get the answer to this and subsequent questions. TIA

 

[AnonymouseUser]

Sorry, you're description is somewhat confusing. Pictures would be helpful, along with model numbers of the modem/router/access point. This forum doesn't allow pictures to be attached to replies, but you can upload them to image sites such as pics.bbzzdd.com (created by Anandtech users for Anandtech users) or imgur.com and post links to them.

 

[matricks]

Below I have made plenty of assumptions, but it sounds right. Your description makes my thoughts very plausible. You can easily confirm it by finding the IP(v4) address of your computer when your PC is plugged directly to the modem, and when it is connected to the "access point". If you don't know what I mean, see How to Find the IP Address of Your PC, and start at method 3 and beyond. Do not use websites that tell you your IP address, they will be misleading in terms of verifying that the problem is what I think it is.

If I'm right, your IP address when connected to the access point will be either 10.*.*.*, 172.[16-31].*.* or 192.168.*.*. These are the possible ranges for private IP addresses. * means any number, [16-31] means any number between 16 and 31 (inclusive). Your IP address when connected to the modem will be none of the suggested patterns above, and can be anything else.

It sounds like the modem simply does what it's supposed to do (modulation-demodulation), and nothing else. It does not route your connection, so whatever device is connected receives a public IP address. The modem (or upstream ISP equipment) remembers the MAC address that was assigned a public IP address. You probably only have one address, so if a second device (different MAC address) requests an IP address, the modem/ISP doesn't respond. Rebooting the modem will unset the remembered MAC address, and let any device request your one IP address.

Your "access point" (make & model, please) probably does routing and NAT, since all devices work until you unplug it (by your description). Any device that connects to your access point will receive a private/internal IP address (which you can have as many of as you like, barring some restrictions up in the millions).

Bottom line is you need a router (which your "access point" is configured as, by the sounds of it). Only the router should be connected to the modem, and all of your devices should connect to the router (directly or otherwise, but not to the modem).

 

[bwbob]

the first thing I see is that you may be actually running 2 routers, the cable modem serves as a router and DHCP's out an IP address when something's plugged into it and the wireless access point is probably also running a DHCP server dishing out addresses to the wireless clients. If they're both set to the same DHCP IP range it definitely cause issues. Check the DHCP settings in each of those to make sure they aren't walking over each other.The cable modem may have a setting that there's a downstream router, the UVERSE one has such a setting, or you can turn off DHCP on the access point and the IP requests will forward through to the cable modem or just set the access point to a different subnet range may be easiest