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Depending on your router, the password is often on a sticker on the router itself (particularly when supplied by your internet company). Look on the sticker that has the serial number, etc. You'll be looking for something like "WEP Key". If you have another router, you'll need to look into your manual for how to login to the router and change it. Typically it is by going to
http://192.168.1.1 in your browser and logging in using the credentials supplied (if it's an Airport basestation, use Airport Utility). If that fails, you can also restore the router to its default settings, which would enable you to use the WEP Key on the side. How to do that can be found by looking in the manual, or if you don't have it, you can locate one online.
