wlan0 hw address changes

[Lucio Crusca <lucio@sulweb.org>]

I need to tell a network admin the hardware address of my wlan0 
interface, so that he gives me access to the wifi network. The problem 
is that the hw address is changing everytime I use the wlan0 hardware 
switch...

# ifconfig wlan0

wlan0: flags=4098<BROADCAST,MULTICAST>  mtu 1500
        ether b2:ad:31:c5:86:36  txqueuelen 1000  (Ethernet) ...

I bring it up using the hardware switch, then

# ifconfig wlan0

wlan0: flags=4099<UP,BROADCAST,MULTICAST>  mtu 1500
        inet6 fe80::c2cb:38ff:fe4c:568f  prefixlen 64  scopeid 0x20<link>
        ether c0:cb:38:4c:56:8f  txqueuelen 1000  (Ethernet) ...

I switch it off again, then

# ifconfig wlan0

wlan0: flags=4098<BROADCAST,MULTICAST>  mtu 1500
        ether aa:c3:d0:e1:43:cb  txqueuelen 1000  (Ethernet) ...

and so on... what is changing the hw address? How do I stop it?