If you're using any version of Windows starting with 2000, Microsoft included an interesting networking tweak called APIPA (Automatic Private IP Addressing).

When a computer on a network first boots, it attempts to get a network address from a Dynamic Host Configuration Protocol server. If there is no DHCP server to be found, Windows happily creates a random IP address in the to range with a subnet mask of Your system then uses this IP address to connect to a workgroup.

Windows continues to search for a DHCP server every few minutes, and when one is located, it will automatically get a lease and drop the APIPA address for the new one supplied by the DHCP server.

While this was an interesting idea to get workgroup computers to still see each other, there were some things the programming team didn't think of. The bad thing about APIPA is you are never notified, unless your computer is Windows XP with service pack 2 installed. This makes troubleshooting confusing for newer techs. They're trained to look for the common denominator, and will overlook the local issues in favor of trying to figure out why the uplink from the workgroup switch doesn't seem to work. If your system can talk to nearby computers, but not the servers or printer pool, it is likely the DHCP server is not available on the local network. Check the DHCP service, restarting it if needed. Clean out old leases and watch to see if it begins to pick up new clients. On some Cisco routers, DHCP can crash and require an IOS reload or a hard power cycle.

Yeah, I said Windows, but this is not part of the quest.