<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:

def gen_mac(prefix='AC:DE:48'):
    '''
    Generates a MAC address with the defined OUI prefix.

    Common prefixes:

     - ``00:16:3E`` -- Xen
     - ``00:18:51`` -- OpenVZ
     - ``00:50:56`` -- VMware (manually generated)
     - ``52:54:00`` -- QEMU/KVM
     - ``AC:DE:48`` -- PRIVATE

    References:

     - http://standards.ieee.org/develop/regauth/oui/oui.txt
     - https://www.wireshark.org/tools/oui-lookup.html
     - https://en.wikipedia.org/wiki/MAC_address
    '''
    return '{0}:{1:02X}:{2:02X}:{3:02X}'.format(prefix,
                                                random.randint(0, 0xff),
                                                random.randint(0, 0xff),
                                                random.randint(0, 0xff))