MAC address filters on access points are easily circumvented using commonly available tools. If your APs are bridged to the Ethernet segment, there are a couple of utilities you can use to look for people spoofing their MAC addresses. Learn about one such tool, arpwatch.
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(Reproduced from the book Wireless Hacks, Second Edition, ISBN 0596101449, Copyright 2006, O'Reilly Media. Reproduced by permission of O'Reilly Media. Written permission from O'Reilly Media is required for all other uses. Visit http://www.oreilly.com/catalog/wirelesshks2/ for a detailed description and to learn how to purchase this title.)