Python script to calculate CIDR values

cidr.py

""" Calculate the subnet mask, its binary representation, number of host and
	network bits, and the total number of hosts for a given CIDR address.
		Usage: python cidr.py [cidr]
		Notes: Pipe the command to jq to pretty print the JSON. Python 2 or 3 compatible.
		Examples:
		python cidr.py 10.0.0.0/24
		python cidr.py 172.0.0.0/16 | jq
"""

from __future__ import print_function

import json
import re
import sys

def netmask(cidr):
	# Regex to represent valid CIDR notation
	valid_ip = "^(([0-9]|[1-9][0-9]|1[0-9]{2}|2[0-4][0-9]|25[0-5])\.){3}([0-9]|[1-9][0-9]|1[0-9]{2}|2[0-4][0-9]|25[0-5])/([0-3]?[0-9])$"

	# verify that the user entered a valid CIDR range
	if not re.match(valid_ip, cidr):
		return json.dumps({'error': 'Invalid IP address'})

	net_bits = int(cidr[cidr.find('/')+1:])
	num_host_bits = 32 - net_bits
	addr_bits = list(('1'*int(net_bits)).zfill(32))
	addr_bits.reverse()
	host_bits = "".join(addr_bits)
	num_hosts = 2**num_host_bits

	mask_vals = [128, 64, 32, 16, 8, 4, 2, 1]

	# how many full octets do we have
	full_octets = int(net_bits/8)
	netmask = '255.' * full_octets
	remaining_bits = net_bits - (full_octets*8)

	if remaining_bits > 0:
		mask = sum(mask_vals[:remaining_bits])
		netmask = netmask + str(mask) + '.'

	netmask = netmask + '0.' * abs((4-(len(netmask.split('.'))-1)))

	# chop off the trailing decimal
	netmask = netmask[:-1]

	# return JSON with all this
	d = {'cidr': cidr,
		 'netmask': netmask,
		 'netmask_binary': host_bits,
		 'num_hosts': num_hosts,
		 'network_bits': net_bits,
		 'host_bits': num_host_bits,
	}

	return json.dumps(d)

if __name__ == '__main__':
	usage = """
Calculate the subnet mask, its binary representation, number of host and network bits,
and the total number of hosts for a given CIDR address.

Usage: python cidr.py [cidr]
Notes: Pipe the command to jq to pretty print the JSON. Python 2 or 3 compatible.
Examples:
	python cidr.py 10.0.0.0/24
	python cidr.py 172.0.0.0/16 | jq
"""

	try:
		input = sys.argv[1]
		mask = netmask(input)
		print(mask)
	except IndexError:
		print(usage)