Ethernet Switch, Unable to identify more than one computer

Are you connecting your switch(es) directly to the modem?  You may need a router in between or a modem/router combo.

No routers here, but, I will order a "Ubiquiti ER-X-US EdgeRouter X 5-Port Advanced Gigabit Ethernet Router" and give that a try. I think I should be able to connect switches to it.

I already tried internet searching "Motorola MBB8600 modem Ethernet network", and similar, and found nothing helpfull. I have found during "xfinity Ethernet Network" searches that others post that they have just used basic unmannaged switches with no issues. I could not find any using my exact same models though. Exact same model should make no difference though. 

That might be your problem.  The modem's ethernet port might allow ONE device to connect, e.g. the first one connecting if you have a 'switch' connected.  Do you have a router you could insert?   Of course, you wouldn't need the switch then unless you have more devices than ports on the router.

Connecting directly to the modem's Ethernet port. It has only one Ethernet port. Both of those switches are the basic unmanaged type. 

I have checked, I beleive, all the reccomended devices. All but two are wifi, and I do not want wifi. One can disable wifi on those devices, but one Can Not Turn Off The Wifi Microwave Output. It is always active, no matter what. I tested that on the Temporarly Rented Arris TG02DCG1682CT hub that the install tech brought. The microwatt / Sq Meter power measurement was exactly the same enabled / disabled. I have already, the other day, ordered the other non wifi device. It has two ethernet ports. But, my Motorola MBB8600 modem has one Ethernet port that is Not Covered Up, and three that are. I peeled off the plastic covering then, connected to one, while a computer was connected to the normal one. It is active, and same issue, just like using a switch. This MBB8600 was sold as a four Ethernet port unit, for some reason, they chose to cover up three of them now. 

The one I placed on order, not arrived yet, is an "Arris SB8200 SURFboard" modem with two  1-GB Ethernet ports. I researched it a lot and could not find verify any instances where someone has connected two computers to the ports. We shall see if it works when it arrives. It is not wasted money since I always have a spare modem on the shelf to help test connection problems. 

There is no doubt about it. You get assigned only one single public IP address from Comcast. That's why you can't hang an ethernet switch directly off of a straight cable modem. Only one device will get an IP address. If you want to use multiple devices simultaneously, you need to use a router hung off of the cable modem. It will assign multiple private IP addresses to the devices as needed.

If others are working with a 'switch' then their Modem is not just a modem, but has integrated router capabilities.  You might check the description of other 'modem' devices offered/recommended by Comcast.  I believe there is a 'gateway' device that is a modem/router combo.

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@Eather wrote:

 

The one I placed on order, not arrived yet, is an "Arris SB8200 SURFboard" modem with two  1-GB Ethernet ports. I researched it a lot and could not find verify any instances where someone has connected two computers to the ports. We shall see if it works when it arrives. It is not wasted money since I always have a spare modem on the shelf to help test connection problems. 

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Only one port will work on the SB8200. Again, a residential Comcast connection gets assigned only one single public IP address.

Thank You. I am sure you are correct. I just ordered a :Ubiquiti ER-X-US EdgeRouter X 5-Port Advanced Gigabit Ethernet Router". When that arrives, within a week it should, then I will reply to the whole thread with the sesults. I think xfinity should find a modem mfr that makes modems that have a basic router on the Ethernet port so people can connect a switch. That then should be in the Approved by xfinity devices list. Maybe there is no such kind of modem though, and so, why is not on list. Making a connection should be as simple as possible for those people who do not have any skills and do not want to develope them, and that likely is most of the people. I learn new skills from this issue, and connecting to xfinity over Ethernet should be simple for me, never an issue again. Thanks. 

Comcast does indeed have several models of combination cable modem / routers. They are called gateway devices. There are several models of retail available gateway devices as well made by popular name manufacturers.