I'm a lazy ass who didn't think it's worth to bother with the standard ipaddress library

ipv6gen.py

import random
import sys

from netaddr import IPNetwork, IPAddress


def generate_random_ipv6(subnet):
    network = IPNetwork(subnet)
    return str(IPAddress(random.randrange(network.first, network.last)))


if __name__ == '__main__':
    if len(sys.argv) not in (2, 3):
        print('Usage: python ipv6gen.py <subnet> <amount>')
        print('Example:')
        print('    python ipv6gen.py 4001:266:f088:1acd::1/64 15')
        print('    (shows a list of 15 random IPs within the given subnet)')
        sys.exit(1)
    
    subnet = sys.argv[1]
    amount = int(sys.argv[2]) if len(sys.argv) == 3 else 10
    
    for _ in range(amount):
        print(generate_random_ipv6(subnet))