Troubleshooting Network Issues

Troubleshooting network issues can be difficult because many times the problem is not local to your computer. You can break down network issues into three categories: those local to the computer, those with the network itself, and those with a network server being used.

When troubleshooting, it's best to first consider local potential points of failure, then move outward because local issues are more likely to be under your control than issues with the WAN.

Local Issues

Check to make sure the settings in Network preferences are set correctly. The Network Status pane provides an overview of the enabled and active port configurations, allowing you to quickly determine which one is having problems.

Network Issues

You should be familiar with the physical topology of your network. Any computer, cable, switch, or router can be a point of failure. When troubleshooting a network, try to isolate the problem by eliminating points of failure. If a computer can reach other computers through switches and hubs, that connection indicates that the physical network is functioning properly. If not, try to work backwards and see if you can reach computers on the same subnet. If not, check your Ethernet cable, and then your Ethernet card to make sure they're functioning properly.

Server Issues

If you have determined that the problem is not local to the computer nor caused by the network, the problem is probably with the server that the user is attempting to use. The server might not be configured correctly or might be down completely.

Through good troubleshooting techniques, you should be able to isolate the cause of the problem. If it is with the network or the server, you might not be able to fix the network or server problem, but you should eliminate local factors before escalating.

Using Network Diagnostics

New in Mac OS X 10.4 is the Network Diagnostics utility that guides you through troubleshooting common networking issues. Some networking applications, such as Safari and Mail, will automatically open Network Diagnostics when they encounter a networking issue. You can open it manually by clicking the "Assist me" button at the bottom of Network preferences, then clicking Diagnostics. You can also open Network Diagnostics in the Finder (/System/Library/CoreServices).

Network Diagnostics automatically runs tests to determine where the problem might be occurring. The results of the tests are displayed as colored indicators on the left side of the window. These indicators will help you determine if the problem is with the local configuration, the network, or possibly a server.

The right side of the screen presents simple questions to diagnose network issues. Based on your answers, it performs additional tests and suggests steps that can be taken to try to resolve the problem. For example, if a user has never connected to the Internet before, it will suggest opening Network Setup Assistant to guide the user through setting up the network. If the computer is connected to a DSL or cable modem, Network Diagnostics might suggest restarting the modem.

Monitoring Network Status

The state of network connections is not static. As connections become active, such as when a PowerBook with AirPort moves within range of a base station, or inactive, such as when an Ethernet cable is unplugged, Mac OS X reconfigures and reprioritizes the network settings to reflect the changes.

You can get a quick overview of the network connection status of your computer by opening Network preferences, which defaults to the Network Status pane. Network Status displays a list of all the currently available port configurations if they are enabled (such as Ethernet, PPTP, VPN, and AirPort). Colored indicators show the activity status of each available port configuration:

• Green The port configuration is active and has been assigned an IP address, either manually or by a DHCP server.

• Yellow The port configuration is active but may not be able to connect to the Internet.

• Red The port configuration is not active.

In addition to colors, the Network Status pane displays a message next to each configuration in the list describing the port configuration's status.

Starting in Mac OS X 10.3.4, Network Status only displays AirPort and Ethernet ports that are both active and enabled. If an enabled AirPort or Ethernet port becomes inactive, such as when an Ethernet cable is unplugged, the message displayed in Network Status announces the problem. After the message displays for a few seconds, the now-inactive port configuration disappears from Network Status. This feature is handy when you're trying to troubleshoot network problems. A quick look at the Network Status pane shows you which ports are active and configured correctly so that you can focus your diagnostic time.

To configure a port, you can select a port and click the Configure button, or choose the port from the Show menu. Choose Show > Network Port Configurations to enable or disable a port configuration.

If a VPN or Modem port is not connected, select the port and click Connect to launch Internet Connect so that you can connect to a network. For example, if you have configured your modem settings in Network preferences but are not connected, select the modem entry and click the Connect button in the Network Status pane. When Internet Connect launches, click the modem's Connect button to connect to your ISP.

Using the Network Status Pane to Monitor Connectivity

To make the most of this lesson, you should have access to a Mac OS X 10.4 computer on a network. Ideally, the network provides Internet access, not just local file and printer sharing services. The exercises in this lesson explain how to set up such a computer, but it's not imperative that you actually do so. You should be able to follow along by reading the step-by-step instructions and examining the screen shots.

The Network Status pane in Network preferences provides a quick overview of the status of each of the enabled network interfaces in order of priority.

1.	Open Network preferences.
2.	Choose Show > Network Status. The Network Status pane lists each of the enabled network interfaces and the status of each one. Assuming you are properly connected to an Ethernet network, the indicator next to Built-in Ethernet should be green, indicating that the interface is active and has an IP address assigned to it.
3.	Unplug the Ethernet connector from your computer. Notice that the indicator in the Network Status pane has changed to red, and the status text states that the cable is not plugged in. After a few seconds the now-inactive port disappears from the list. If your Ethernet cable is plugged in to an inactive port, for example on a switch that is broken or unplugged, you will see similar behavior.
4.	Plug the Ethernet connector back into your computer. Notice that the indicator in the Network Status pane returns to green.