Monday, January 10, 2011

Windows 7: IP Address Conflict -- Another Computer Has Same IP Address

(As always, with these long posts where I am working through a veritable cluster of issues, feel free to use Ctrl-F to search for the exact error message that brought you here, if any.  Otherwise, these long posts are provided to stimulate ideas, for those who have no solutions as they work with intractable networking problems.)

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I was installing Windows 7 on two separate computers.  As described in a previous post, I was having problems with networking.  This post picks up the discussion partway through that process, when I received this message:

Network Error

Windows has detected an IP address conflict

Another computer on this network has the same IP address as this computer.
In response to this error, one person suggested just powering off the router and then starting it back up.  I had been doing that and was getting tired of it.  Besides, I felt that we might really be on to something here -- that this was, at least, a possible clue as to my network problems.  Another essentially suggested going into Control Panel > Network and Sharing Center > Change adapter settings > double-click on Local Area Connection > Properties > select Internet Protocol Version 4 (TCP/IPv4) > Properties ... but my settings there were already automatic, as suggested -- and also to make sure, in the router's setup webpage, that DHCP was enabled and no machines were being assigned a static address.  But another disagreed, saying that the IP address should be set manually.

Another said, "This issue is usually caused by resetting a router without resetting all the network connected devices."  And that was possible.  I had not yet tried his suggestion, which was to power down everything on the network, including all computers, network printers, modem, everything; then turn on the modem and wait for a steady data light; then turn on the router and wait two minutes; then turn on the other devices and computers, one at a time.  So I did that.  That did not make the router work.  I tried using a network switch instead, a Netgear FS605.  On the switch, the old Win7 computer worked, but the new one did not.

Another suggestion was to type "ipconfig /release" and then "ipconfig /renew" in a command window on each computer.  I tried that last step.  On computer A, the steps worked.  On computer B, I got this:
Windows IP Configuration

An error occurred while releasing interface Local Area Connection: : An address has not yet been associated with the network endpoint.
I decided to try installing fixed IP addresses on these two computers.  Back in TCP/IPv4 Properties, I changed from "Obtain an IP address automatically" to "Use the following IP address," and I set the values as follows:  IP address =, Subnet mask =, Default gateway =  The IP address was identical to that from the other computer except the other ended with 64.  The Subnet mask was provided automatically, when I started to fill in the boxes, and the Default gateway was the same as on the other computer.  I also checked "Validate settings upon exit."  When I exited out of that, Windows Network Diagnostics offered to automatically update my network settings, saying, "Windows can detect the correct network settings for you."  I chose, "Apply this fix."  The problem, as it then explained, was that until then, "DHCP is not enabled for Local Area Connection."  That, it said, was now fixed.  But "ipconfig /release" still said no address has yet been associated with the network endpoint.  I tried rebooting.  The problem was still there.  My changed settings were gone; the setting had been returned to "Obtain an IP address automatically."

The situation at this point seemed to be that I had to connect to the network -- that is, I had to have the two computers connected together, through switch or router -- in order to have both computers obtain an IP address; but somehow I had to persuade them to get their IP addresses before the system identified an IP address conflict and thereupon shut down the networking capabilities of the router, the switch, or one of the computers.  I wasn't sure this was quite right, but there seemed to be some kind of lose-lose, reciprocal defeat process going on there.

I decided to look at the error message (above) that I was getting when I tried running "ipconfig /release."  The message said, "An address has not yet been associated with the network endpoint."  A search unfortunately gave me a lot of pages I had already checked, suggesting that I was running out of options here.  I did find a thread that suggested several possibilities.  One was that antivirus or firewall software might be "blocking the machine from picking up DHCP from the router," suggesting that maybe I would need a working router with antivirus software temporarily disabled if I hoped to resolve this problem.  Eventually, I switched from AVG antivirus to Microsoft Security Essentials.

Another suggestion from that thread was to install Tomato or DDwrt firmware in the router, assuming I could get it unbricked or find a replacement, because the Linksys WRT54GL (same as mine) allegedly "has lots of complaints," not working consistently with Windows 7, at least until better firmware was installed.  The concept there was that I would connect the router to one computer, upgrade the firmware, and then connect it to the other computer, and maybe in that way the computers would become so adjusted and sane, in this crazy world, that they would be able to cooperate even if I did later substitute the switch for the router.  In that particular thread, unfortunately, a firmware upgrade did not save the day.  Another possibility was that, if I had Bonjour installed (as part of e.g., iTunes or Adobe CS5), that would cause problems.  But I hadn't installed any such program software at this point.

It seemed that I should be able to do the process with one computer before doing it with the other.  I tried that.  With one computer connected directly to the modem, I took the recommended steps:  type "ipconfig /release" in a command window; manually enter and Tab past the Subnet mask in the TCP/IPv4 Properties; save that; go right back into TCP/IPv4 Properties and switch to "Obtain an IP address automatically"; save that; and then enter "ipconfig /renew."  But when I ran "ipconfig /all" to see what had happened, I saw "Media disconnected."  So maybe the computer was confused.

At this point, I returned to this post after an absence of several days.  I plugged computer B into the network switch.  That worked.  Then I unplugged computer B from the switch and instead plugged in computer A.  After a pause, that worked too.  Now, I plugged them both into the switch.  As above, I got the "IP address conflict" error on computer B.  After a few minutes, I got it on computer A too.

This problem had been lingering for a while, and I was not clear at this point what steps I had tried previously.  Influenced by what may have been a redundant search, I found a thread and a more clearly written webpage that summarized most if not all of the various things that people tried to resolve this problem.

Trying one of those steps, I disconnected computer A from the switch.  Then, going back through those steps, on computer B I found, first, that "ipconfig /release" gave me the "network endpoint" error (above).  At that point, the switch had ceased to work.  Even though computer A was no longer connected to it, computer B was still unable to go online.  I removed the switch from the equation -- that is, I connected computer B directly to the modem -- and tried again.  Now computer B was able to go online.  I tried "ipconfig /release" again.  I got the same "network endpoint" error.  I now realized that this probably meant that it was already released and there was nothing else to release.  So I tried "ipconfig /renew."  Its output gave me only a blank for "Connection-specific DNS Suffix."  Evidently that was OK:  I was able to go online.  I disconnected computer B and connected computer A directly to the modem, tried the same steps, and got the same result.  Now I powered up the switch and connected both computers directly to it.  After a few minutes, computer A again popped up an "IP address conflict" error.

With only computer B connected to the switch, on both machines I tried a command-line approach to set static IP addresses.  The steps here were to type these two commands in a cmd box (Start > type "cmd"):
When I tried that on computer B, I got an error:  "The requested operation requires elevation (Run as administrator)."  A search led to the advice to type this to open the cmd box in administrator mode:  "runas /user:administrator cmd."  This didn't work.  The reason, as I found when I ran that command inside another cmd box, was that "Blank passwords not allowed."  My administrator account had no password.  Trying another approach, I switched to computer A, where I was already logged in as administrator (and therefore would not interrupt or confuse the file-copying process that was underway on computer B), and tried the same commands there.  The second command ("interface ip set ...") returned an error:  "The configured DNS server is incorrect or does not exist."

interface ip set dns “Local Area Connection” static
While it was tempting to investigate the command-line option, I went back to the main webpage and found it easy to agree with the advice that "If you don’t need to use a static IP address, it’s best to simply choose DHCP instead of manually configuring the IP address."  This advice had the virtue of not requiring me to do anything.  Unfortunately, it had the drawback of leaving me where I started.  I tried some other links from the search, but now found that computer A was not connected.  Apparently the netsh command had screwed it up.  I typed "exit" at the netsh prompt and closed the cmd box, but this was not good enough.  So I reopened the cmd box and tried "ipconfig /release" and "ipconfig /renew."  That wasn't the answer.  I still could not go online on computer A.  I could have rebooted it, but I had a file-copying process underway there too.  (I was just setting up these computers.  Data had to be rearranged.)

So, OK, I waited until the file copying process was done on computer B, connected its cable to the switch instead of computer A's, and tried to log in as Administrator over there.  I had not previously done that.  Unfortunately, the Administrator account was invisible.  To see it, I had to find a way to enable it.  One approach to do that was to open an elevated command prompt, which was what I probably needed to do above, but now I had seemingly better instructions.  The approach I took was to start by creating a shortcut to an elevated command prompt:  right-click on the Desktop (the actual desktop, not the link in Windows Explorer) and choose New > Shortcut > type "C:\Windows\System32\cmd.exe" for its location (i.e., the target of the shortcut) and call it "Elevated Command Prompt."  Now I followed the instructions to open the elevated command prompt (via the shortcut) and type "net user administrator /active:yes."  The response:  "System error 5 has occurred.  Access is denied."  A search for that seemed to indicate that my elevated command prompt was not giving me elevation.  Basically, system error 5 just meant access denied.  Someone suggested the old "control userpasswords2" command, but that didn't seem to be functional anymore in Windows 7.  Another approach (for Win7 Pro, Ultimate, and Enterprise) was to go to Start > type "netplwiz" > Advanced tab > Advanced user management > Users > double-click on Administrator > uncheck "Account is disabled" > Apply > OK.  Now Administrator was visible in Control Panel > User Accounts.  I rebooted and went into the Administrator account. 

Now that I was logged in as Administrator, after an intervening lunch and nap, I found that I had no idea why I needed to be an Administrator.  I sure wasn't going to enter that apparently destructive netsh command again.  But upon reviewing the previous paragraph, I saw that I was intending to type "net user administrator /active:yes" at an elevated command prompt. I would have thought that my user account, which User Accounts showed as being an administrator account itself, would have sufficed.  Apparently not.  Surely now, though, as an official Administrator, I would be able to go ahead and enter that net user command.  And indeed I was, and I did.  This time, no System Error 5.  Instead, "the command completed successfully."  Good.  Now what?  I decided to try "ipconfig /all."  This confirmed my recollection that computer B was showing no "Primary Dns Suffix" and no "Connection-specific DNS Suffix."  I tried a search on related terms.  The consensus seemed to be that the blank space was not a problem on a standalone machine.

Back at the lucid instructions webpage, it appeared that I had tried everything.  I wondered if perhaps there was a hardware problem with the switch -- if maybe the computers had screwed up both my router and my switch.  Possibly unbricking the router and doing a firmware upgrade, using just one computer, would make a difference.  I plugged computer B only into the router.  It was still a brick.  A search for unbricking led to a suggestion that I make sure it was actually bricked before I tried to unbrick it.  A recommended solution for this was to go to its internal webpage.  For my Linksys WRT54GL v. 1.1, that was, so with computer A being the only thing connected to the router, I typed that address in the address bar in my browser and hit Enter.  The default username was blank; the default password was "admin."  That brought up the internal page.  This was not entirely the behavior of a brick, was it? 

I had previously posted the settings I used in the router.  Those settings were still there.  I was using a wire-only network (i.e., not wireless).  Following the manual for the Linksys WRT54GL router (whose instructions were repeated in brief form on the router's internal webpage), I had to change from DHCP to PPPoE in the router's Setup > Basic Setup tab.  Changing to PPPoE opened up a bunch of other options, including my AT&T username and so forth.  Next, on the Wireless > Basic Wireless Settings tab, I set Wireless Network Mode to Disabled.  I made sure Security > Firewall > Filter Internet NAT Redirection was turned off.  In the Administration > Management tab, I changed the router password, and changed to https, and disabled Wireless Access Web and Remote Management.  UPnP sounded like it was going to make life easier, so I left it enabled.  Finally, I went into the router's Administration > Config Management > Backup > Save to make a backup of the current configuration.  Then I went into Administration > Firmware Upgrade tab.  I browsed to the latest firmware upgrade, which I had previously downloaded, and then clicked Upgrade.

So now I wanted to see if these settings and the firmware upgrade would change anything.  First, I connected computer A directly to the modem.  It still couldn't go online, and by now I had rebooted that computer, so apparently that netsh command had really screwed up something.  I went into Control Panel > Network and Sharing Center > Troubleshoot problems > Internet Connections > Troubleshoot my connection to the Internet.  It said, "Your computer appears to be correctly configured, but the device or resource (primary DNS server) is not responding."  A search on that message turned up very little information other than my own post from a few days earlier.  One post said something about using OpenDNS servers instead of those of the poster's own ISP.  Well, that was interesting.  I should use a non-AT&T server with my AT&T account?  It seems OpenDNS had a free service of some sort.  The concept seemed to be that you could "unbundle" your DNS service from your ISP.  Apparently AT&T was now my primary DNS server.  So I could spend a half-hour on the phone with AT&T, or I could try OpenDNS.  I had a feeling I would probably wind up doing both.  But here I was with OpenDNS, so why not?  I've written that up in a separate post.  It seemed to solve the problem.  If it was the netsh command that was to blame, then it appeared that OpenDNS was somehow able to circumvent it in a way that AT&T wasn't.

Alright.  So computer A was able to go online via direct connection to the modem.  Now, how about through the router?  I connected only computer A to the router, and the router to the modem.  But no.  There was no action here.  I ran Win7's Internet Connections troubleshooter again.  This time, a different error:  "Your broadband modem is experiencing connectivity issues."  I turned off the power for "at least 10 seconds," as the troubleshooter recommended.  Oh, but wait.  I was unplugging the *router.*  They were talking about the *modem.*  The modem was having connectivity issues?  If so, how was I able to use it with computer B to save these words in this blog post?  But let's not argue.  I did what they said, this time doing it to the modem, not the router.  I not only unplugged it, I also went on to "press and quickly release the Reset button" on the modem.  I clicked Next on the troubleshooter, but this gave me an ominous remark:  "The connection between your access point, router, or cable modem and the Internet is broken."  Yikes!  I looked at the modem and, sure enough, a big red light where there should have been a green one.  I disconnected the router from the modem and connected computer B to the modem, and gave the modem another ten-second rest.  But no.  When it came back on, I still had the big evil red one.

See, I knew this was going to come down to another phone call to AT&T somehow.  I knew it.  I felt it in my bones.  So, alright, I called AT&T tech support.  I got their automated guy.  He always sounded a bit exasperated with me, like he was explaining this to an idiot.  Fair enough.  But he ended by telling me that I had to check my connections, reboot, and then call back within 24 hours.  End of conversation.  He was polite enough about it, but still.  I guess I couldn't complain.  Instead of spending a half-hour to get a possibly correct answer, I got no answer almost immediately.  This could be a good deal, where AT&T was concerned.

Well, I got off the phone and finished typing and saving these words on computer B, using the connection where the AT&T automated guy had detected a problem.  (Computer B had been the one that was connected while he and I were chatting.)  Oh, the big red one went away when I did another power cycle.  I probably just hadn't unplugged it for long enough.  I had time to redo it while the automated guy was asking me about who I was and what I wanted.

A while later, when computer A had finished doing something else that made it unavailable for my purposes (I felt like a pest, always wanting something just when these machines were up to their elbows in more important challenges), I rebooted and connected it directly to the modem once again and saw that it was now able to go online without a problem.  I ran the Internet Connections troubleshooter again, just to be sure I wasn't halllucinating.  Sure enough, it detected no problem.  That was unacceptable, of course, but fortunately I knew how to fix the situation.  I connected computer A to the router, and the router to the modem, and gave it another whirl.  Got that message about the modem experiencing connectivity issues.  It occurred to me to disconnect computer A from the router and connect computer B there.  I ran the Internet Connections troubleshooter on computer B.  There, where I had not done the transition to OpenDNS, I got the message about not being able to connect with primary DNS server.  I re-ran this comparison, using the switch instead of the router, one last time.  Here, both computers connected without a problem.

To review:  the first problem seemed to be that I had a nonfunctioning router.  The ability to connect to its internal webpage did not mean that it was functioning for purposes of connecting with the world.  The troubleshooter messages pointing to the modem seemed to be mistaken.  The second problem was that I had an IP address conflict.  This seemed to be something that a functioning router might have fixed.  I had worked through other possible solutions without success, so that seemed like my last hope.  At the same time, I was concerned that possibly the IP address conflict itself had screwed up the router, and my use of the switch had suggested that the router, like the switch, would have no effect on the IP conflict.  I could hope that somehow this problem derived from the AT&T modem, though it was not clear how.  The other thing that had arisen, which may have been a real issue or maybe not, had to do with DNS.  I decided not to install OpenDNS settings on computer B.  Perhaps future experiences would teach me about the difference between AT&T and OpenDNS in that area.

Just in case I hadn't already done so, I did a reset.  I could have done this by holding the reset button for five seconds.  I wasn't sure about the time period -- I had heard a number of different periods of time for pressing reset buttons, by this point -- so instead I went into the router's software at and went to Administration > Factory Defaults > Restore Factory Defaults > Yes > Save Settings. I was pretty sure I had tried this before, but I tried it again now. When that was done, it required me to log into the router again, which meant a blank username and "admin" as password. Then I connected the router to the modem and waited to see if I could go online. I couldn't. I went into Control Panel > Network and Sharing Center > Troubleshoot problems > Internet Connections. The lights on my DSL modem were all green, which was good. But the troubleshooter said, "The conection between your access point, router, or cable modem and the Internet is broken." It asked me to do a modem reset, which I had done previously.

So the action steps at this point seemed to be to debrick the router, to ask AT&T if they had any ideas and, failing that, to some up with some other approach to the IP address conflict.  This seemed like a dismal list of action steps, rather oriented toward failure.  I did call AT&T again, but since I was able to connect the computer directly to the modem and get online that way, they were only able to offer fee-based support with what seemed to be a Linksys router problem.  And I would have gone that route, if I had been convinced that I would get my money's worth.

I ran another search.  Regarding the router, one post arising from this search made me think I should try setting the router to DHCP rather than PPPoE.  I did that.  It did not make a difference.  I switched back to PPPoE and notice, as I had noticed previously, that PPPoE contained a "DHCP Server Enable" option, which was checked.  So probably DHCP made no difference.  Looking at a post from the previous search re: debricking, it seemed that my router was not bricked if I could still contact its web base menu and if its power light was not continuously blinking.  Another webpage took me through a more elaborate set of steps and criteria.  I have explored that whole process of unbricking and updating router firmware in a separate post.  It took hours and yet did not solve the problem.

I had been reluctant to throw money at the problem, especially if there was a risk that the network situation itself had been responsible for damaging the router.  It was suspicious that it had been working so well, during several periods over the past several years, and suddenly fell apart when I introduced Windows 7 networking to my computing world.  It was also suspicious that the problem occurred likewise on the network switch, which had likewise seen several years of service.  Yet there was a difference:  computer B was able to go online through the switch, but not through the router.

These developments suggested a revised set of action steps.  One was to see if a replacement router allowed at least one computer to go online.  Another was to continue to work on the IP address conflict, though I was out of ideas.  A third was to think further about the differences between the two computers, and the possibility that a screwed-up situation on computer A was causing problems for the network as a whole.

The plan emerging from those steps was to buy a replacement router, but to test it on computer B first, and see whether it, like the switch, would allow computer B to go online.  Meanwhile, it appeared increasingly likely that I would have to try reinstalling Windows 7 on computer A, to see if that made a difference.  I would probably wait until I had done that before trying to connect the new router to computer A, just in case the Windows installation itself was somehow confusing or damaging the router.  I hoped that a new Windows installation on computer A would work with the switch and with the old and/or new routers.

That seemed to be something I could test without too much difficulty.  I made an image of the current state of Windows 7 on computer A, in case I wanted to keep it, and then started a new installation on computer A.  The networking impact of a new Windows 7 installation is covered in another post in this blog.


Laurie M

ok, you got me going, great writing style, for a normally dry and boring subject, I am having the same problems, but, you Left Me Hanging! To be continued next episode, what happened? Did it work with a new router, with re-installing windows 7, come on, you can't just leave it like that! This is like a soap opera, All Too True


Sorry, Laurie M -- Blogger has only now allowed me to approve your comment, 17 months after the fact. I hope you did stay tuned for subsequent episodes in this saga.


What I've got from problem like this is this : adapter not enabled for dhcp. My solution is usually I turn off my router/switch for 1-2 minutes and I turn on again after that, and all computer in network don't have IP Adress Conflict warning. It often happens in my workplace, and that's what I've done each time the IP Address Conflict warning came.