from sys import argv
import argparse
import ipaddress
import random
#import getopt #option function

parser = argparse.ArgumentParser(description="Generating IPv6 networks")
parser.add_argument('--size', help="size")
parser.add_argument('string', nargs='+', help="network")

try:
    args = parser.parse_args()
    
    if argv[1] == '--size':
        ipnet = argv[3]
        size = 128 - int(argv[2])
    else:
        if len(argv) > 2 :
            ipnet = argv[1]
            size = 128 - int(argv[2])
        else:
            ipnet = argv[1]
            size = 80

    ip = ipnet.split('/')
    ip2 = ipaddress.ip_network(ip[0]).supernet(new_prefix=int(ip[1]))

    # minimum network
    minNet = ipaddress.ip_network(ip[0]).supernet(new_prefix=128-size)

    # seperate network
    ip2first = ipaddress.IPv6Network(ip2)[0]
    ip2last = ipaddress.IPv6Network(ip2)[-1]
    minNetlast = ipaddress.IPv6Network(minNet)[-1]

    # calculation network
    maxRan = int(ip2last) - int(minNetlast) >> size
    result = ipaddress.ip_address((random.randrange(0,maxRan) << size) + int(ip2first)  )
    resultNetwork = ipaddress.ip_network(result).supernet(new_prefix=128-size)

    print(resultNetwork)
    
except:
    info="""
wrong input  
ex) --size 64 2001:db8::/40 
    or 2001:db8::/40 64
    or 2001:db8::/40  #default size 48
"""
    print(info)