Most Ethernet adapters have two or three status indicators that light and go dark as data moves through the network connection. On an expansion card, the lights are usually on the metal mounting bracket just above or below the RJ-45 connector; on a PC card, they might be on the card itself, or on the socket that connects to the network cable. Indicator lights on a built-in Ethernet socket are usually right next to the socket itself.
The three indicator lights are:
Lights in green when the adapter is connected to a live network
Lights in yellow when the adapter is connected to a 100 Mbps network; goes dark when it's connected to a 10 Mbps network
Flashes in green as data passes through the network connection in either direction
These lights are useful for troubleshooting and watching network performance, because they can tell you whether your network connection is working properly. For example, when the LINK indicator lights, you know that the computer is connected to a live network; when it's dark, the network connection is offline. When the ACT light flashes, it tells you that the computer is sending or receiving data.
Unfortunately, the lights are often located in a place where it's difficult or impossible to see them while you're using the computer. When you're testing the system, it's often helpful to ask someone else to watch the lights while you operate the computer.
On some computers, the LINK and 10/100 lights might remain on when the computer is turned off. This happens when the computer's BIOS has a "wake on LAN" feature that turns on the computer if the adapter receives an incoming signal. Even if this feature is disabled, the computer keeps the network port alive whenever the power supply is turned on. The only way to disable the network adapter completely is to either unplug the AC power or turn off the power switch on the back of the power supply.