Each Network card connected to a pc, whether it be wirelessly, GPRS, wired (ethernet) or anything else has a MAC address, its an address that identifies that particular card. Whether it has a valid IP address or not.
Most routers/hubs and even in some cases switches can tell which MAC address is which and block them from accessing parts of the network, only allowing certain known Network cards in, this does create some good security... In theory. Of course MAC addresses can be cloned, and therefore that is all security gone, unless it's encrypted
Well, i hope you understand what i mean, it will simply reject you any access other than connecting, somtimes it will not even let you connect.