When you attempt to connect to the Internet, you may see alert messages such as: “The server could not be found.” “The specified server could not be found.” “A connection failure has occurred.” “Connection attempt failed.” “Name servers not responding.” Despite the alert, some Internet (TCP/IP) applications may function normally. Tip: In Mac OS X v10.4 or later, you can click the “Assist me” button in the Network preference pane, then follow the onscreen prompts for network diagnostics.
Source:Troubleshooting "A connection failure has occurred", "The specified server could not be found" or similar messages
