Tuesday, September 30, 2008

Tech Support call about 169.254.x.x

I had an interesting call today from someone trying out the NetScanTools Pro 10 Demo. He wanted to know if it could help him figure out why his laptop was getting a 169.254.x.x IP ( subnet mask) when he plugged it into his work network. He was aware of the Microsoft Windows XP default action of using Automatic Private IP Addressing (APIPA) to assign an IP in that range whenever DHCP failed. But he wanted to confirm that DHCP was not working and just talk to someone about it I guess.

As a way of confirming my suspicions, I had him use the DHCP Discover Tool in NetScanTools Pro 10 Demo. When he pressed the Discover button, the DHCP server returned no data and the tool timed out. I had him do this a couple of times. The tool sends out a DHCP_DISCOVER message and looks for responses much like ipconfig does -- and there were no responses.

So either his physical network connection was bad or the DHCP server was dead. He was pretty sure his network cable was OK, so that left the DHCP server itself. I had him confirm this using ipconfig /release, then ipconfig /renew, then confirmed his computer was set up to use DHCP by looking at the results of ipconfig /all. Now that he was pretty sure of the program, he had a call into whoever was responsible for the DHCP server.

I also had him go to Network Statistics to check to make sure there was only one network interface in his system and he could see all the parameters for the single interface like IP, mask and MAC address. That way he could be sure that there were no other conflicts like a wireless and a wired interface.

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