MAC the Knife

A good preventive measure that stops a hacker from gaining access to
your WLAN is to keep a MAC access control list. This list identifies the
unique code of each networking card. The server or access point can be
easily programmed to either grant or deny access to an incoming network
connection based on whether or not that code is in the acceptable
list. Most hackers gain access because this feature is not enabled by
default. In an open networking environment, anyone can gain access to
your system at any time. The MAC access control list (ACL) screens out
any unauthorized user attempting to access your WLAN. This simple
measure is a very effective means of protection.