#!/usr/bin/python3

from socket import AF_INET6, getaddrinfo, inet_ntop, inet_pton
from argparse import ArgumentParser, ArgumentDefaultsHelpFormatter

parser = ArgumentParser(formatter_class=ArgumentDefaultsHelpFormatter, 
	description='concat ipv6 addr with given host and prefix part')
parser.add_argument('host', help='ipv6(name) for the host part')
parser.add_argument('prefix', help='ipv6(name) for the prefix/network part')
parser.add_argument('prefix_len', nargs='?', default='64', type=int,
	help='prefix length')
args = parser.parse_args()

pa = lambda s: inet_pton(AF_INET6, getaddrinfo(s, None, AF_INET6)[0][4][0])

addr = pa(args.prefix), pa(args.host)
res = [0, ] * 16

for i in range(128):
	res[i//8] += addr[args.prefix_len <= i][i//8] & 1 << (7 - i % 8)

print(inet_ntop(AF_INET6, bytes(res)))