Symptom
You see the network "Up" flag in the Apple System Profiler or Terminal, but you do not have a valid network connection on that network interface (port).
For example: There is no cable attached to the built-in Ethernet port, but the "Up" flag still appears alongside other built-in Ethernet (en0) network interface information.
Solution
Verify a network connection by checking physical connections or accessing other devices on the network (or, if applicable, accessing the Internet). You may be able to use the Network Utility (/Applications/Utilities/) to verify connectivity.
For related information, see these technical documents:
106749: "
Mac OS X: Troubleshooting a Cable or DSL Internet Connection"
106260: "
Mac OS X: Computer Name Does Not Appear on Network"
61426: "
Mac OS X: About Network Utility"