Several years ago, I was a host of the Cajun Clickers Computer Club’s weekly radio show on WJBO in Baton Rouge. We ran a call-in "ask the experts" show about computers on Saturday mornings.
One of the calls was from a lady who had multiple wireless computers. Originally, they all would connect successfully to her router. Now, some of those in a different part of the house wouldn’t connect any more.
After some question and answer, it all broke down to wireless security. You see, she had never set up any security! She was just using the wireless router straight out of the box. Without the security setup, her neighbors’ wireless routers were interfering with hers.
One of the most important things you can do for your wireless system is to make it secure. You configure both the router and the computer that’s going to connect wirelessly to the router.
By doing this, not only do you keep others out of your system (and keep your files and printer, and your ISP account, free from interference), you are also definitively telling your computers WHICH router to which they should connect.
In this case, the lady had a DLink router, and she was using the default SSID "DLink." But, some of her computers were showing that they were connecting to "Belkin." In other words, because of signal strenght or interference, they were trying to connect to a neighbor’s wireless router. As often happens, the computer will lock in on that foreign signal but then be unable to get an IP address from it because of the router’s security settings. So, they end up with NO networking!
My article Wireless Security discusses the steps to take to protect your network — and to make sure it keeps working.
By the way, you can read more about the Cajun Clickers Computer Club at its web site.