Friday, January 14, 2011

Windows 7: IP Address Conflict: Maybe It's the Modem!

I had been wrestling for days with a home networking problem.  The problem was that either of my two computers could go online, by itself, when I would connect it to the DSL modem directly or through a network switch; but when I would plug them both into a network switch or router, neither would go online or connect with each other.  Plugging both in at the same time would quickly produce an error message:

Network Error

Windows has detected an IP address conflict

Another computer on this network has the same IP address as this computer.
Most recently, I had explored the possibility that the problem was with a corrupted Windows 7 installation on computer A.  I had reinstalled Win7 on computer A with a brand-new installation; but as soon as I connected it to the switch, both computers were once again unable to go online.

I didn't want to go through the Win7 setup process again, so I replaced that new installation with one that I had made a day or two after the initial installation.  Eventually, computer A, with this day-old installation, did give me the Network Error message.  Interestingly, though, computer B did not.  It was now able to stay connected, through the switch, while computer A was failing to connect.

I had a laptop, running Vista.  It had been a while since I had tried to connect it to this wired network.  I connected it directly to the modem.  It was able to go online that way.  I connected it to the switch while computer B (but not computer A) was connected.  The laptop reported an IP address conflict.  This suggested that the problem was with computer B.  To test that, I removed computer B from the switch and plugged in computer A and the laptop.  But no, those two had a conflict as well.  Finally, I plugged in the laptop and a Windows XP computer.  Those two had an IP address conflict as well.  So it was not some kind of flaw with Win7.

This brought me back to a problem that I had considered previously.  I had seen, somewhere, an indication that the DSL modem was responsible for assigning IP addresses.  I had followed up on this partway:  I had called AT&T, and they had told me that, as long as a direct connection between the computer and the modem was successful, they could not provide me with any further free assistance.  I had had the option of paying for assistance, and at this point it seemed I should have taken that option, because now I was thinking that the modem was failing to assign IP addresses.  Well, but OK, now that it was diagnosed, I wondered if I could fix it myself.

I had bought this modem from AT&T about two and a half years earlier.  My previous notes on it did not detail the setup process.  I think maybe AT&T took care of that for me.  I had moved to another place; the modem had spent some time on a shelf.  But now it was back.  I looked on the bottom.  Nothing there specified a model number, but I guessed that's what the number 2210-02-1022 was.  It said it was "Style MSTATEA."  Motorola didn't seem to know anything about it.  At first, there didn't seem to be a user's guide, but then I figured out a search that got me somewhere.  The manual that I found said there was a web-based configuration page, which I could find at  I went there.  It showed me a basic Connection Information and Modem Information and Local Network screen.  Here, I saw that the modem's model number was 2210-02 and I was using software version 7.7.5r8.

At the left side of that screen, I clicked on Login.  It said, "You are currently connected to the Internet."  I went into Advanced > Connection Configuration.  It asked for the Modem Access Code printed on the bottom of the modem.  Apparently this was secret information, so I decided not to type it in this post.  I entered the code.  Evidently I didn't need the login information I had gotten from the manual.  The manual said the default username was "admin" and the password was something that I created on the spot.  It already had the login information for my AT&T account there.  I remembered that a tech support guy had instructed me to set PPPoE as the protocol; that was still there.  The manual (page 9) said,
Your Modem serves Dynamically-assigned IP addresses by default.  Be sure to configure each computer connected to your Modem to accept a Dynamically-assigned IP address, commonly referred to as DHCP.
The instructions, translated into Win7 terms, essentially had me go into Control Panel > Network and Sharing Center > Change adapter properties > Local Area Connection > Properties > TCP/IPv4 > Properties > Advanced.  There, it looked like DHCP was set already, as the manual said.  Trying something else, I went into the router's Advanced > PPP Location option.  It said, "PPP is on the modem."  I had an alternative:
PPP is on the computer, gateway or router. This should only be used if you need to run a PPPoE client on your PC or you use another device (e.g., gateway or router) to initiate a PPPoE session. This is often referred to as "Bridged" mode.
I tried that.  The router's internal webpage gave me these further instructions:
PPP Location Warning


When using Bridged mode, your access to the modem becomes limited. To return to the DSL modem user interface after this change you need to directly connect your PC to the modem without any gateway or router between the modem and the PC, and configure your computer appropriately.

Configure the IP address of your computer to be on the same network as the modem by using an IP address of the form 192.168.1.x (except and a network mask of

You may also return to the DSL modem user interface by resetting the modem back to its initial defaults. All configuration changes and other settings will no longer be available if this is done. To reset the modem press the "Reset" button located on the back of the modem.

Modem Restarting

The modem can be used after the power light has been steady green for at least 15 seconds.
That approach might have worked if I had still been using my router, but I wasn't.  I was using a network switch, and it wasn't playing ball.  Apparently switches weren't capable of doing the bridge thing.  I reset the modem (didn't have to connect directly; a switch was apparently like a straightpipe for that purpose).  Now, in the modem's Connection Information screen, I got back an Internet IP Address and Internet Gateway Address after a few seconds.  Now that I had gotten over my panic and saw that I could rescue the situation, I actually read the instructions and saw that I hadn't tried the part about configuring the computer's IP address to be 192.168.1.x.  So I went back and did it all again, and this time I did that part too, back in the same location as above ( Control Panel > Network and Sharing Center > Change adapter properties > Local Area Connection > Properties > TCP/IPv4 > Properties.  There, I replaced the automatic settings with the numbers shown above.  Still didn't work.  Windows Network Diagnostics started up automatically.  After a minute or two, it said it couldn't identify the problem.  So I guessed that maybe I had it right the first time, that I needed a router instead of a switch.  But that didn't make any sense.  I had used this switch with this modem before.

So, alright, the joke was on me.  I had to pay AT&T to make the same switch work with the same modem, where they had worked without a problem before.  In the meantime, while I was on hold, I tried to set the modem back the way I found it, but now I wasn't able to connect.  So that was good.  AT&T would have to help me with that.  I could get some more free service out of them before I had to pay them.  And I did.  The guy was very helpful at getting me back to where I started.  It took an hour.  Then he passed me on to AT&T's paid service (877-831-2880).  The phone there rang and rang.  Evidently AT&T wasn't so eager to take my money.  No canned voice giving me information on what number I had reached, or anything.  But at least no hold while listening to godawful music specially selected to discourage people from calling AT&T free tech support.  After ringing for several minutes, I hung up and tried that number again.  This time I did get a canned voice.  So apparently he put me through to the wrong number.  This, he said, was ConnectCheck.  It allowed me to choose tech support.  It rang and rang.  So maybe he was right after all.  He said they would be there until, oops, maybe the wrong time zone.  Friday night, too late, probably would have to wait until Monday.  AT&T, I love you!

So let us review.  How did the guy get me back to working order?  I think we mostly just did ordinary things.  He had me reboot into Win7's Safe Mode with Networking and see if I could browse.  I could.  So he thought maybe the problem there was with antivirus software.  We didn't pursue that back in Normal Mode, though.  He had me do hard reset on the modem, and then log into the modem's webpage, but I think we just looked at information there.  He really had no idea what a switch was; it took him a while to wrap his head around the concept.  Ultimately, he and his managers concluded that the IP Address Conflict error had to be coming either from a bad switch or from bad drivers.  Did I need drivers for the switch?  It was a Netgear FS605 v3.  Well, there was an installation guide.  A manual!  RTFM!  Fortunateliy, there was no installation process or software.  Just plug it in, which I had done.  So how about his other theory:  a bad switch?  Anything was possible, but what had happened on my system -- recurrently, it would seem -- that would have wiped out, first, my router, and then my switch?  Was AT&T sending out electromagnetic pulses to drum up customers for its paid support line?  I could get a replacement for $25 including tax at Sears, or a few bucks cheaper online if I didn't need it right away.

While I was talking to the guy, I plugged in my Vista laptop again.  It came up with a different error message than before.  In addition to the one about an IP conflict, it gave me an option to "Automatically get new IP settings for the network adapter "Local Area Connection."  It also advised turning off the modem or router and unplugging its power cord and then replugging after 10 seconds.  I tried the automatic solution first.  That immediately disappeared after I tried it.  It offered an option to "Reset the network adapter "Local Area Connection,'" so I tried that next.  That failed too, so I went on to unplug the modem, and simultaneously did likewise with the switch, and then replugged them sequentially, waiting for the modem to show solid green on four out of five lights before replugging the switch.  Most of these were steps that I had tried many times before at this point.  They didn't make any difference.

At this point in the game, pending paid support from AT&T which wouldn't materialize until Monday (this was Friday night), I decided that the modem was not the problem.  But what was?  That question called for a new post in this continuing saga.