How to calculate netmask from 2 ip adresses in Python

Question

How can I calculate the subnetmask in Python if I have the first and the last ip adresses in a range?

I want the netmask as eg 255.255.255.0.

Thanks ;)

Answer 1

Say that we have...

def ip_to_int(a, b, c, d):
    return (a << 24) + (b << 16) + (c << 8) + d

Then you can have the representation doing a few XORs. Eg.

>>> bin(0xFFFFFFFF ^ ip_to_int(192, 168, 1, 1) ^ ip_to_int(192, 168, 1, 254))
'0b11111111111111111111111100000000'

So:

def mask(ip1, ip2):
    "ip1 and ip2 are lists of 4 integers 0-255 each"
    m = 0xFFFFFFFF ^ ip_to_int(*ip1) ^ ip_to_int(*ip2)
    return [(m & (0xFF << off)) >> off for off in (24, 16, 8, 0)]

>>> mask([192, 168, 1, 1], [192, 168, 1, 254])
[255L, 255L, 255L, 0L]