Wednesday, August 18, 2010

How to diagnose an IP conflict

I haven't posted for awhile but here's a good one. I just had a situation where only one computer could access the internet and not only that, but if any other computer was turned on, both or all got a message about an IP conflict. I think the message was something like "another computer has the same IP" and there was a yellow triangle. Now, the computer that was first turned on could access the internet but the second could not. In trying to figure out the problem I attempted to access the router (type in the address bar and hold the Ctrl button and press enter) and came up empty "the process timed out". I tried different combination's of the last two numbers and got into the modem instead. Then I got smart and hit Start>all programs>accessories>command prompt and typed in "ipconfig" and saw that the IP issued to my computer was close to the IP issued to modem. This was a clue because the network in your house only has a couple of reserved address ranges and this was out of range. I decided to have a look at the router and found the power light blinking while Mary informed me that there had been a power surge or loss earlier in the day. I unplugged the router and waited 1 minute and plugged it back in and the power light quit flashing.
After this, any computer turned on was able to access the internet and go about their business.

Explanation: the router quit working and the modem went default and issued the next IP address to the first computer that was turned on. The modem is not a router and can't handle multiple IP's. Once the router was working again it resumes its duties. The clue was the abnormal IP address at the computer level. When behind a router you should expect an address starting with "192.168.x.x or 10.x.x.x or even though obscure 172.16.x.x .

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