Finding largest netmask for given set of hosts

Edvard Majakari edvard.majakari at
Fri Aug 9 07:23:47 EDT 2002

I'm trying to construct a class method which returns the smallest possible
network (largest netmask) for given set of hosts.

Now, I have list of ip addresses in object variable self._list. Each IP 
address is in binary format (32-bit long)

The algorithm is below:

    def gen_mask(self):

        nw = self._list[0]      # initialise network to first ip address
        nm = 0xFFFFFFFFL        # initialise netmask to (32)

        # for every other ip in the list
        for ip in self._list[1:]:
            nw = nw & nm  # AND network with netmask

            # XOR the network with ip address. All differing bits
            # should be ones after XOR, and all bits that are same should
            # be ones

            x = nw ^ ip 

            # now, if nw was and first ip was,
            # result of x should be 24L, which is corresponds to bitmask of
            # 00000000000000000000000000011000. However, this is
            # undesirable - we need a bitmask that has ones for the
            # constant parts of the ip addresses and zeroes for the rest.
            # by inverting x (~ x) we get
            # 11111111111111111111111111100111, which still contains
            # one at the end of the bitstring. We get rid of this bit
            # by calling a function that zeroes all the rest bits after
            # the first zero bit:
            nm = zero_after_first_zero(dec2bin(~x))

            # now nm (according to the example) is 
            # 11111111111111111111111111100000. We use this to 
            # to recalculate new network:
            nw = nw & nm

            # now nw should be and netmask is 27

        return nw

The problem is that in Python, it seems quite difficult to create a
routine dec2bin that would handle binary digits. I also doubt the
algorithm is the best for this kind of thing. 

I wonder is there any existing Python modules that would do this kind of
thing? I searched one for days (using Google), but didn't find any..

# Edvard Majakari		Software Engineer
# PGP PUBLIC KEY available    	Soli deo gloria - Glory to God alone!

$_ = '456476617264204d616a616b6172692c20612043687269737469616e20'; print
join('',map{chr hex}(split/(\w{2})/)),uc substr(crypt(60281449,'es'),2,4),"\n";

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