I came in to my office this morning and the entire network was 'down'; meaning that no computer could establish connectivity to another computer on our network, and no one could get out to the internet.
The network consists of a few unmanaged netgear switches and a sonicwall firewall. I finally tracked the problem down to one specific machine: our Windows 2003 server running as a primary domain controller. When it is plugged into the network no one can ping anyone else, but when it is unplugged, the entire network functions properly. I tried disabling the network adapter in order to use the machine's second network adapter. It wouldn't disable though. I restarted the server and tried the original bad adapter with the same results. I was finally able to disable the first network adapter and enable the second one with the same IP config. Voila, everything is up and running just fine.
My question: has anyone ever run into something like this and what could cause it? What sort of things should I Look for? Unfortunately I can't really troubleshoot with the first adapter more during the work day, so I have time to compile troubleshooting tips and try after hours.