Is it plugged in?
Let’s assume you are called in to look at a system on your professional media network. The operator tells you, “I can’t access anything on the network.” Where do you start?
We are all familiar with the first questions you are typically asked when you contact computer technical support: 1) “Is it plugged in?” 2) “Is it turned on?” When you are troubleshooting, this is the first place to start: Does the Network Interface Card (NIC) indicate that you have a good electrical connection? Usually, you can verify this by looking for an LED next to the place where the network connection plugs into the network device. No light, no joy. Period. In almost every case, if you do not have a link light, there is something physically wrong with a cable or a connector. In some rare cases, you will not get a link light because something in the networking hardware did not initialize properly during the boot sequence. In any case, you will have to find and fix the problem if you do not have a link light.
If you have a link light, the next step is to see if the device’s operating system (OS) recognizes that a network connection is present. (See Figure 1.) The LED is a low-level indication that a link has been established. If network card drivers are damaged or improperly configured, the OS will not be able to access the network even though the link light is lit.
Many network engineers prefer to use a command line interface to troubleshoot network problems. If you are not comfortable doing this, I would strongly recommend that you spend time learning how to access a command prompt on whatever troubleshooting systems you use, and that you become familiar with a few simple commands. It will save you time.
The commands ifconfig or ipconfig will show you the status of all of the network interfaces on your device in *NIX and Windows, respectively. Look for the status or media state entries. If you see “media not connected” or “inactive,” these indicate that, as far as the OS is concerned, no network exists. (See Figure 2.)
In almost every case, when you see this indication, you also do not have a link light, so the trouble is either physical or in a low-level driver. Plug a laptop into the same cable. Do you get a link light? If yes, then you may have a driver problem on the computer. If you do not get a link light, then you know that either the cable or connector is likely to be bad.
If you have a link light, and the status of the connection is “active,” check to see if the computer has an IP address that makes sense. You probably know the IP address range of the network, the gateway and the netmask. If not, you can either find someone who can give you this information, or you can go to another computer on the same network, open a command line interface and enter either ifconfig or ipconfig, depending upon the OS.
Computers get IP addresses in two ways: Either someone manually configures the computer (a likely scenario in a professional media network), or the computer gets its address from a Dynamic Host Control Protocol (DHCP) server. In any case, if the computer does not have an IP address, has an IP address that does not match the host network, or does not have gateway and network entries that make sense, then the computer will not be able to access the network, even with an active network connection.
One common problem is not being able to “pull” a network address from a DHCP server. There are many things that could cause this problem, but without the correct IP address, netmask and gateway entries, your computer will not be able to operate correctly.
What if you have a link light, the status of the NIC is “active” and you have valid network parameters assigned to the NIC, but you still cannot access the network? Well, now it is time to get a little more specific about what “access” means. If this is a computer on the Internet, you can try using the ping command. At a command line interface, if you enter ping followed by a name or IP address, your computer will make repeated attempts to contact the remote device. If everything is working normally, the remote device will reply, and you will see the results on your command line. But, note well that, many times, remote hosts are configured to generally ignore ping requests.