These days, nearly every network adapter has an associated number called a MAC address. This number is (almost always) unique and is defined in the network card hardware—making it permanent and unchangeable. Thus one aspect of wireless network security is to only allow access to devices with specific MAC addresses. And while this is a good practice, it will not keep out a determined hacker. Despite what I wrote above, MAC addresses can be easily spoofed at the operating system level. The linked article explains how to do it in Windows. This is another reason why there is no such thing as total network security without encryption (and even then, it has to be the right type of encryption).