Se não me engano, deve ser o Endereço mostrado ao emitir
sudo iwlist scanning
Por exemplo:
wlan0 Scan completed :
Cell 01 - Address: 00:11:22:33:44:55
Channel:5
Frequency:2.432 GHz (Channel 5)
Quality=70/70 Signal level=-33 dBm
Encryption key:on
ESSID:"the_known_ssid"
Bit Rates:1 Mb/s; 2 Mb/s; 5.5 Mb/s; 11 Mb/s; 18 Mb/s
24 Mb/s; 36 Mb/s; 54 Mb/s
Bit Rates:6 Mb/s; 9 Mb/s; 12 Mb/s; 48 Mb/s
Mode:Master
...