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I could think of half a dozen places this could start this conversation. I don’t believe the problem is hardware related, but the information that I am getting from my customer doesn’t convince me that it is vendor (AT&T DSL) related either. I maintain a small office with 5 computers which are connected to a switch. The switch goes to a router and the router goes to a DSL modem. The user has a database that lives on a server on the LAN. The user complains that “they are always getting kicked off the internet”.
That being said, during my last trip to the office, I made a sketch of all the connections and lights. The user understands the lights on the modem (power, connectivity, etc.), lights on the router (power, etc.) and the lights on the switch. The problem is, so they report, they are kicked off, but all of the lights are still green. Scratch your head and ask more questions.
Do they have any problem talking to the database during the time they cannot communicate with the internet – the answer is not consistent. In general it seems to be 50-50.
I replaced the switch last week, allowing that the switch was old (10/100) and could explain why users couldn’t talk to the server. If they couldn’t get through the switch, they wouldn’t be able to get to the internet or the server. The problem has continued.
The user is not afraid to turn off the modem and router, and then power them up again. And in general terms, once he does this, his connectivity has been restored for a period of time. He has had to power cycle the modem and router 4 times today, and the day is not over. (He comes in at 8:30 am).
Does anyone have any suggestions? Where would you start looking? What would you replace next. (I will likely replace the router next, but we are seriously considering going to a cable connection to the internet.

Unless all of the other four users/computers are having problems I wouldn't consider replacing anything hardware related, except MAYBE the cable running to the computer in question or the networking card on said computer. I wouldn't suggest throwing any more money at the situation yet. Troubleshooting this scenario in a hardware sense requires a different computer, (hopefully one of yours that you know has ethernet capabilities) and a different ethernet cable. (hopefully one that you know to be reliable) I do not proscribe hardware replacement (or even troubleshooting) as a first course of action in this case for two reasons: operating a toaster is more difficult than power-cycling a modem or router, and technological illeteratti are far more common in offices than faulty cables and networking cards are.

If ONLY one computer is having problems with being kicked off and all of the other ones are fine, it could be the computer, it could be the user of that computer, or it could be a configuration problem. I have a feeling that you really have no way of finding out for certain.

THE VERY FIRST THING THAT I WOULD DO is troubleshoot the electricity being provided to the modem, router and switch.

Make sure that the modem, router and switch are plugged into a surge-protected power strip.

Plug a lamp into the same power strip. Then try turning everything in the office on. Open the refridgerator door, turn on all the computers, turn on the microwave, the hotplate, the electric heater and a/c. Try to get everything on that breaker running simultaneously. If that breaker services another office, try to get them to turn all of their electrical gadgetry on, too. If you can't get that breaker to trip, at least see if that light will flicker. I wouldn't be surprised if you quickly trip that breaker with all the stuff people have plugged into it.

If you can get the breaker to trip, or even get the light to flicker noticeably you definitely have a suspect. Then you can solve the problem by changing where some of the high-powered stuff plugs in, (like heaters, microwaves, refridgerators, especially the stuff that causes big spikes) or change where the networking devices plug in, or buy a UPS or call an electrician. Or all of the above.

If it really is ONLY one computer having the problem, it could be a dynamic vs. static ip configurations in modem, router, switch (if its a managed switch) and the computer itself. (doubtful) It could also be some malware changing occasionally changing proxy settings or other configurations. (very doubtful)

It could also be other users in the office changing router settings to meddle with hapless user for stealing their creamer out of the fridge. Checking the router's firmware settings is an EXCELLENT idea if you haven't already, NO MATTER WHAT THE PROBLEM IS. If you don't know what your router's firmware interface is, either tell us what model the router is or let the internet be your guide.

But... this really really really smacks of power problems.
Usually pretty easy to fix. Be crative. Just don't do it dangerously :)

What the previous poster said. If all the systems are having problems and you replaced the switch, then it is either a router problem, or a modem problem. Since the router and modem are usually provided by the ISP, contact their tech support to get a service tech on site. One of my clients had the same sort of problem recently, and it turned out to be a bad router/modem. They replaced it and everything was back to "normal".

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