Host computers connect to a wired network using a network interface and RJ-45 Ethernet cable. Most network interfaces have one or two LED link indicators next to the interface. Typically, a green LED means a good connection while a blinking green LED indicates network activity.
If the link light is not on, then there may be a problem with either the network cable or the network itself. The switch port where the connection terminates would also have an LED indicator lit. If one or both ends are not lit, try a different network cable.
Note: The actual function of the LEDs varies between computer manufacturers.
Similarly, network infrastructure devices commonly use multiple LED indicators to provide a quick status view. For example, a Cisco Catalyst 2960 switch has several status LEDs to help monitor system activity and performance. These LEDs are generally lit green when the switch is functioning normally and lit amber when there is a malfunction.
Cisco ISRs use various LED indicators to provide status information. A Cisco 1941 router is shown in the figure. The LEDs on the router help the network administrator conduct some basic troubleshooting. Each device has a unique set of LEDs. Consult the device-specific documentation for an accurate description of the LEDs.