Protocol dysfunction or misconfiguration— The protocol software doesn't work or (for whatever reason) isn't configured to operate properly on the network.
Line problems— A cable isn't plugged in or isn't working. A hub, router, or switch isn't working.
Faulty name resolution— DNS or NetBIOS names can't be resolved. Resources are accessible by IP address but not by hostname or DNS name.
Excessive traffic— The network appears to be working, but it is working very slowly.