Subnet_mask

# Verify the accuracy of a input subnet mask
while True:
    alw = [255, 254, 252, 248, 240, 224, 192, 128, 0]
    mask = raw_input('Enter the subnet mask: ')
    m = mask.split('.')

    if len(m) !=4 or (int(m[0]) !=255 or (int(m[1]) or int(m[2]) or int(m[3])) not in alw) :
        print 'Error: this is not a subnet mask\n'
        continue
    '''
    # Is it the right condition ?
    if int(m[1]) < 255 & int(m[2]) == int(m[3]) == 0:
        pass
    if int(m[1]) == 255 & int(m[2]) < 255 & int(m[3]) == 0:
        pass
    if int(m[1]) == int(m[2]) == 255 & int(m[3]) < 255 :
        pass
    '''
    # Why It doesn't work as well?
    if (int(m[1]) >= 255 or int(m[2]) != int(m[3]) != 0) & (int(m[1]) != 255 or int(m[2]) >= 255 or int(m[3]) != 0) \
       & (int(m[1]) != int(m[2]) != 255 or int(m[3]) >= 255):
        continue
    break