It is possible to 'spoof' a MAC address (i.e. change the MAC address) for some hardware. If someone found out one of your MAC addresses, they could simply set their hardware to use that MAC address and get onto your network. People sniffing packets may also be a problem. As all the data is being sent unencrypted, there is the possibility people could read that data pretty easily without being on the network.
Set up WPA-PSK encryption on all your hardware, if it supports it (if not, go for WEP). Give it a very complicated and secure password - you will only need to enter it one time on each PC, so it doesn't really need to be so simple you can remember it easily. Make the password the maximum possible length and include lots of special characters if you can.
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