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An Unexpected IP


Some material is very old and may be incorrect today

© August 2009 Anthony Lawrence

This call happened to be for an old SCO Unix system, but really it could have been for anything: Windows, Linux, Mac OS X, it would have all been the same. An old server had an OS transplant and needed to be connected to the network.

The consultant at the other end of the phone had already run through "netconfig" (SCO's network configuration tool) without luck. I asked what kind of card it was and was told that it was an Intel Pro 1000 and that he had downloaded drivers from Intel.

I felt we ought to try the drivers from SCO's ftp site instead. I don't know that there's anything wrong with the drivers at Intel - they might be the exact same drivers for all I know - but he wasn't having any luck so far, so why not? We did, and after installing them and all that, the network still didn't work. An "ifconfig net0" showed that everything was fine, but he couldn't ping anything.

I had to ask if he was sure that everything was OK physically. No chance of a bad wire, dead switch port? No possibility of a link speed speed mismatch? No, he said, he had booted from a CD and had network connectivity. As we'll see in a moment, that couldn't have been precisely true, but when troubleshooting remotely we have to go with what we're told until we learn differently, so...

"Though that was DHCP", he said.

Hmmm. Well, OK. Maybe he's confused about the IP he's using - it could already be in use. So sure, let's try DHCP. Why not?

After that reconfiguration and reboot, I asked for an "ifconfig -a" before we tried anything and got a surprise: the ip was 172.16.12.12. Why was that a surprise? Because earlier he had been trying to assign a 172.16.2 address - dot 2, not dot 12.

That wasn't a mistype - he wanted to be on the 172.16.2 network. Obviously he was not. Apparently he was plugged into the wrong switch port. Although he was unaware of it at this time, the network was segmented using a smart switch and whatever port he was using was on the 172.16.12 section.

As he was previously unaware of that network, I now suggested that he needed to sit down with the client's IT people and find out where he really needed to plug in. He said that would probably be the server room, which makes sense: the poor machine was out of its native element. Take it home where it belongs and everything would probably be fine.

So there we left it. I showed him how to configure a default gateway and pointed him at articles for setting resolv.conf and populating /etc/hosts. That should do it.


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