networkAdds = addresses.splitlines()[1:]
networkAdds = set(add.split(None, 2)[1] for add in networkAdds if add.strip())
with open(sys.argv[1]) as infile:
    knownAdds = set(line.strip() for line in infile)
print('These addresses were in the file, but not on the network')
for add in (knownAdds - networkAdds):
    pass
Python get list of mac addresses and compare them with list from file