First, you need to find out the MAC address of all of the wireless adapters on your PCs.

1. Open a command prompt on each computer, type ipconfig /all, and press Enter.

2. The screen that appears will display a good deal of information. Look for the numbers next to Physical Address, such as 00-08-A1-00-9F-32. That's the MAC address. Write all those MAC addresses on a piece of paper.

Now log back into your router, and configure MAC address filtering. On the Linksys WRT54GX4 router:

1. Click Wireless, and then click Wireless Network Access to get to the Wireless Network Access screen.

2. Select "Permit only," and type in the MAC addresses into the text boxes. Click "Save settings." Now, only computers you specify will be allowed onto your network.

Turn off your network when you're not using it

This simple precaution can go a very long way toward keeping you safe: Simply turn off your router when you're not using your network. If you're off at work all day and no one's home, why keep your router running? The same holds true for when you sleep, or are away from your house for another reason. The less time your wireless network is available, the less likely it is to get hacked.

Check for wireless intruders

You can never be too safe, and so even if you've taken all this advice, it's a good idea to check your network to see if intruders have made their way in. And if you haven't taken all this advice, that's all the more reason to check.

In the next part of this two-part article, I'll show you how you can check your network to see whether any intruders have managed to worm their way in.

Editor's note: Do wireless nets really pose that much of a security threat? Computerworld editors Preston Gralla and David Ramel take opposing views on this question in a Sound Off. Read Preston's "Why you need wireless protection" and David's "Why worry about wireless?" and then then weigh in on the issue with your comments and in our QuickPoll.

Preston Gralla is a contributing editor to Computerworld.com and the author of more than 35 books, including How the Internet Works, (Que, 2006).