If you're getting ready to update to the Windows XP Service Pack 2, Microsoft has just released a guide on application compatibility. If you're concerned about which applications you might have problems with, it's worth a look.

For most home users, installing and running with Windows XP Service Pack 2 is not a problem. However, some Internet applications may need specific ports opened in the new Windows Firewall. Last week we showed how to tell the firewall about new applications, but sometimes you also need to specify additional ports.

In our example here, we are going to open ports for the remote control program PCAnywhere by Symantec. PCAnywhere uses two ports, one TCP for data and one UDP for control. For it to work, PCAnywhere needs to communicate between two systems on these ports. While the SP2 Firewall may not block outgoing traffic it will block incoming requests.

To open a port, click on Start and control panel. In Control Panel, select the Windows Firewall icon. In the Windows Firewall dialog box, (Figure 6), select the Exceptions tab. At the bottom, select Add Port. In the Add Port dialog (Figure 7), enter a name, and the port number. Click either TCP or UDP, and click OK, which takes you back to the Exceptions tab. If you need to do another, click Add Port again, and repeat.

When opening ports for computers on an internal network, you may want to limit the addresses that can access the newly opened port. To do this, click on the Change Scope button when adding a port. The Change Scope option (Figure 8) allows you to limit the range of IP addresses that can access the port. Once you've added all the ports, click on OK from the main Firewall dialog and exit.