convert cidr to subnet mask in python

Question

I have a python dictionary that I've created, this dictionary contains a list of subnets in the following format:

x.x.x.x/24
y.y.y,y/25
z.z.z.z/26
a.a.a.a/27

etc...

I would like to take the items in this dictionary, parse it, then spit out the results in the following format:

x.x.x.x 255.255.255.0
y.y.y.y 255.255.255.128
x.x.x.x 255.255.255.192
a.a.a.a 255.255.255.224

I don't have much on this as of right now because I can't find a lot on this topic on the web, not anything that can be in a quick and concise way that is. Thoughts?

If it helps to see an example, I made a class that does this [here](https://github.com/adamacosta/OMSCS-6250-Examples/blob/master/iputils/iputils.py). — Adam Acosta, Nov 17 '15 at 06:13

score 15 · Answer 1 · edited Sep 27 '19 at 03:50

15

Code:

import socket
import struct

def cidr_to_netmask(cidr):
    network, net_bits = cidr.split('/')
    host_bits = 32 - int(net_bits)
    netmask = socket.inet_ntoa(struct.pack('!I', (1 << 32) - (1 << host_bits)))
    return network, netmask

Usage:

>>> cidr_to_netmask('10.10.1.32/27')
('10.10.1.32', '255.255.255.224')
>>> cidr_to_netmask('208.128.0.0/11')
('208.128.0.0', '255.224.0.0')
>>> cidr_to_netmask('208.130.28.0/22')
('208.130.28.0', '255.255.252.0')

edited Sep 27 '19 at 03:50

Trenton McKinney

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answered Nov 17 '15 at 06:39

falsetru

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Goes without saying, but for the IPv6 initiated. CIDR is the official notation for IPv6, there for netmask is not a thing anymore. But if you still wanted to, you could do it: https://gist.github.com/Torxed/148303f1677e56dcca794386e90299fc – Torxed Jan 07 '22 at 15:40

score 11 · Answer 2 · edited Sep 27 '19 at 03:50

I thought I'd throw in my own solution, since I was going for a little bit more readability in it than the other answers shown.

def cidr_to_netmask(cidr):
  cidr = int(cidr)
  mask = (0xffffffff >> (32 - cidr)) << (32 - cidr)
  return (str( (0xff000000 & mask) >> 24)   + '.' +
          str( (0x00ff0000 & mask) >> 16)   + '.' +
          str( (0x0000ff00 & mask) >> 8)    + '.' +
          str( (0x000000ff & mask)))

It's now easier to see what's going on, and that is:

Get the numeric mask by padding the front with 1s and having the cidr make up the rest
For each bit, apply the mask
Concatenate all the bits together, separated by periods

This is very procedural and does not use any libraries.

João Paulo Andrade · Answer 3 · 2018-11-09T19:51:08.330

10

Try this solution:

Python3

from ipaddress import IPv4Network

networks = {'n1':'10.1.0.0/21','n2':'10.2.0.0/22','n3':'10.3.0.0/23','n4':'10.4.0.0/24'}

for x,y in enumerate(networks):
    print(IPv4Network(networks[y]).network_address, IPv4Network(networks[y]).netmask)

Result:

10.1.0.0 255.255.248.0
10.2.0.0 255.255.252.0
10.3.0.0 255.255.254.0
10.4.0.0 255.255.255.0

Python2

from netaddr import IPNetwork

networks = {'n1':'10.1.0.0/21','n2':'10.2.0.0/22','n3':'10.3.0.0/23','n4':'10.4.0.0/24'}

for x,y in enumerate(networks):
    print(str(IPNetwork(networks[y]).network), str(IPNetwork(networks[y]).netmask))

Result:

('10.1.0.0', '255.255.248.0')
('10.2.0.0', '255.255.252.0')
('10.3.0.0', '255.255.254.0')
('10.4.0.0', '255.255.255.0')

edited Nov 09 '18 at 19:51

answered Nov 09 '18 at 19:28

João Paulo Andrade

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`apt install python-netaddr` – xinthose Dec 06 '18 at 22:54
https://pypi.org/project/ipaddress/ is a "Port of the 3.3+ ipaddress module to 2.6, 2.7, 3.2" in case you still need to maintain Python2 code and want to use `ipaddress`. – yaobin Jan 06 '22 at 21:51

score 0 · Answer 4 · answered Nov 17 '15 at 06:28

Try this

lsIP = []
ans = 0
CIDR = 32
IP = [1] * CIDR
for i in range(len(IP)):
    iIdx = i % 8
    if iIdx == 0:
        if i >= 8:
            lsIP.append(ans)
            ans = 0
    ans += pow(2, 7 - iIdx)
lsIP.append(ans)

[lsIP.append(0) for i in range(4 - len(lsIP))]
print lsIP

convert cidr to subnet mask in python

4 Answers4

Code:

Usage:

Python3

Python2

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