Invariably, troubles and misconfigurations creep into networks. New devices get connected and added to a network. Old devices are removed, and something seemingly unrelated breaks. Troubleshooting is really a test in discerning patterns. My favored method for solving problems is to start with the simplest elements, verifying correct operation and proceeding to the next layer or element until I have isolated the problem element. If you are lucky, you'll know from a symptom where the problem is likely to be, but more often, you'll have to start at the bottom of the networking hierarchy, and verify each other layer.

The first thing to consider whenever somebody reports a strange networking problem is any recent change. What has changed recently in the network? Have any new machines been added? Is the user using a service which was recently decommissioned? Did a machine (firewall, mail server, DNS resolver) recently reboot? Did all of the services restart?