>>>Hi,
>>>
>>>I have been connecting my computer to internet via Wi-Fi (router) for a long time. And I just changed the ISP from Comcast Business to Comcast Home. So, I got a new Comcast modem. The person in the Comcast office assured me that the speed of the Comcast Home will be as good if not better than Comcast Business. This kind of works sometimes, and does not work other times. I have not yet figured out what is the problem, when there is a problem.
>>>
>>>Meanwhile I decided to try connecting the computer to the Comcast modem directly, using Cat 5 cable. But, after I connect the computer to the modem with Cat 5, and disconnect from wireless, I have no internet connection. How can I tell or test if my computer is connected to the Comcast modem directly?
>>
>>I would run the network troubleshooter in Windows first, if you haven't already.
>
>First check firewall enabled and hosts file - without router there is no NAT and/or firewall (except perhaps/probably windows-own) between him and attackers out in the net. In W2K times usually 20s before somebody starts sniffing for openings.
>
>Check rubber suit first before entering unknown regions... somewhere else proxies could be hiding ?

I admit I made some implicit assumptions about the OP's environment.

In North America these days, residential "modems" provided by ISPs are almost always combination devices of modem + router, also usually with WiFi on the router. It's been quite a while since I've seen a new residential device that was just a modem. Some ISPs even provide those for business customers, in that case they can at the business' request change them to a pass-through non-routing mode so they can be used with customer's existing routers etc. But pass-through mode is not the default, especially for residential.

So if that's true of the OP's device, there should be a router in place.

That said, if the OP's previous device *was* just a modem, and he's using a separate router, then replacing the modem with a combined modem/router could cause issues.

If Comcast did the installation, any tech will typically test that the connection is functional while on-site, and reconfigure the supplied device if necessary to work properly with existing customer equipment. OTOH Comcast is legendary for crappy customer service...