Get IP Mask from IP Address and Mask Length in Python

Question

Given an IP Address in dotted quad notation, for example:
192.192.45.1
And a mask length for example 8, 16, 24 typically, but could be anything i.e. 17.

Can somebody please provide the code in python to calculate the subnet mask? Preferably I could get the result as 32-bit integer so that it is easy to hash and then reinterpret as dotted quad when necessary for printing. I see that python has a socket library which is basically a wrapper around the unix socket api. I also saw it has the function inet_ntoa(), but it returns some sort of packet struct. I'm not terribly familiar with the Python struct library, so I was hoping some other would have some ideas. Thanks!

Answer 1

The simplest way is to use google's ipaddr module. I assume a 25 bit mask below, but as you say, it could be anything

>>> import ipaddr
>>> mask = ipaddr.IPv4Network('192.192.45.1/25')
>>> mask.netmask
IPv4Address('255.255.255.128')
>>>

The module is rather efficient at manipulating IPv4 and IPv6 addresses... a sample of some other functionality in it...

>>> ## Subnet number?
>>> mask.network
IPv4Address('192.192.45.0')
>>>
>>> ## RFC 1918 space?
>>> mask.is_private
False
>>>
>>  ## The subnet broadcast address
>>> mask.broadcast
IPv4Address('192.192.45.127')
>>> mask.iterhosts()
<generator object iterhosts at 0xb72b3f2c>

Answer 2

You can calcuate the 32 bit value of the mask like this

(1<<32) - (1<<32>>mask_length)

eg.

>>> import socket, struct
>>> mask_length = 24
>>> mask = (1<<32) - (1<<32>>mask_length)
>>> socket.inet_ntoa(struct.pack(">L", mask))
'255.255.255.0'