A problem with a network hub or cable is not really a TCP/IP problem. However, you can still use TCP/IP diagnostic utilities such as ping to diagnose line problems. In general, if the network used to work and has stopped working suddenly, a line problem is often the cause. Make sure all network cables are properly plugged in. Most network cards, hubs, and routers have display lights that indicate whether the unit is on and ready to receive data. Each port of a hub, router, or switch has a link status light that shows whether an active network connection is operating through the port. Several tools exist for testing network cabling. If you don't have access to a cable testing tool, you can always unplug a suspicious cable and plug a new cable into its place to see if that solves the problem.
You can use ping (described in an earlier section) to isolate line problems. If a computer can ping its own address but cannot ping any other addresses on the network, the trouble might be in the cable segment that connects the computer to the local subnet.