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Why would an IP address not release from an old computer after it was taken down and off the network?

Let me explain what happened... I set up static IPs for all of my computers and when replacing an old computer, give the new one the old IP. So computer "A" was assigned 192.168.1.150

This computer had to be replaced, so it was shut down, taken completely off the network and put to the side.

Computer "B" is then brought up onto the network, assigned the IP 192.168.1.150 but was reporting that there was an IP conflict. So the computer kept dropping from the network and nothing would work properly until I changed the IP address.

I then gave the new computer about 12 hours until I changed the computer back to the old IP again (for testing sake) but still reported IP conflict.

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5 Answers 5

up vote 4 down vote accepted

This might be a problem with the ARP cache in the router you are connected to. I have had to clear out the ARP cache in such cases.

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That seems like a logical solution. What makes it even more weird is that it's the first time it has happened on over 50+ computer replacements. –  C-dizzle Jan 9 '13 at 18:33

Like mdpc said, probably ARP cache. The router is seeing that same IP trying to correspond with 2 MAC addresses, which is what a router sees when there is an IP conflict. Clear the ARP cache manually. If it's a home router and you have limited options, just do a factory reset if it persists.

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In addition to clearing the arp cache on routers, try pinging the IP address in question and then looking at the local ARP cache to find the MAC address of the machine in question.

Assuming this is a Windows Machine, you can view the ARP cache from a command line using these commands -

c:>arp -a <--- This displays the local arp cache

This can help you track down the machine in question if you indeed have a machine online with that IP address.

-Chris

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Could you please verify that when the IP address of the computer B is set to something different than 192.168.1.150 you are unable to ping some device with the IP address 192.168.1.150. It could be that there is some device that you do not know about with the same IP.

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If you're using dynamic addressing with DHCP, the lease might not expire and be still unavailable for other devices. If you're not using DHCP, then it is probably ARP cache as suggested above. You could also erase ARP cache and let the machine or a router do arp discovery again. Windows machine http://www.techrepublic.com/blog/window-on-windows/quick-tips-flush-the-arp-cache-in-windows-7/4271

Particular commands to clear ARP cache on a router will vary depending on the manufacturer and the model of the device. Here's how you could do that on a Cisco router: http://www.cisco.com/en/US/docs/routers/crs/software/crs_r4.2/addr_serv/command/reference/b_ipaddr_cr42crs_chapter_010.html

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