old_contents: from django.core.exceptions import ValidationError
  import re
  
  
  mac_pattern = re.compile(r'^([0-9a-f]{2}:){5}[0-9a-f]{2}$')
  
  def validate_mac(mac):
      if not isinstance(mac, basestring) or not mac_pattern.match(mac):
          raise ValidationError('Invalid MAC address')
  
new_contents: from django.core.exceptions import ValidationError
  import re
  
  
  mac_pattern = re.compile(r'^([0-9a-f]{2}:){5}[0-9a-f]{2}$')
  
  def validate_mac(mac):
      if not isinstance(mac, basestring) or not mac_pattern.match(mac) \
              or mac == '00:00:00:00:00:00':
          raise ValidationError('Invalid MAC address')
  
subject: Make 00:00:00:00:00:00 an invalid MAC address