def lookup(mac_address)
      load_prefix_file! if @prefixes.nil?
      oui = mac_address[0..7].upcase.gsub(':', '').to_sym
      @prefixes[oui] || 'Unknown'
    end
def lookup(mac_address)
      load_prefix_file! if @prefixes.nil?
      oui = mac_address[0..7].upcase.gsub(':', '').to_sym
      @prefixes[oui] || 'Unknown'
    end
Public: Get organizational name for a MAC address

 mac_address - A MAC address with format: hh:hh:hh:hh:hh:hh

 Example:

     Nmunch::MacOuiLookup.instance.lookup('58:b0:35:31:33:73')
     # => Apple

 Returns name of organization or Unknown if prefix can't be found