How to generate a random IPv6 subnet in Python 3

I have an existing IPv6 address block. How can I generate a random /64 network in this block using Python, preferably without using any external libraries?

As an example, you can start from the fd00::/8 Unique Local Address (ULA) block to generate a random private ipv6 network.

Solution

You can do this using the standard random and ipaddress modules. Short version:

from ipaddress import IPv6Network
import random

ula = IPv6Network("fd00::/8")
random_network = IPv6Network((
    ula.network_address + (random.getrandbits(64 - ula.prefixlen) << 64 ),
    64))

Longer version with explanation:

from ipaddress import IPv6Network
import random

# Start from the ula address block
ula = IPv6Network("fd00::/8")

# Get a random bitstring the size of the number of bits we can randomise.
# This is the number of bits reserved for the network (64) minus the number of bits
# already used in the address block we start from (8).
random_bits = random.getrandbits(64 - ula.prefixlen)

# Bitshift those bits 64 times to the left, so the last 64 bits are zero.
random_address_suffix = random_bits << 64

# Add those bits to the network address of the block we start from
# and create a new IPv6 network with the modified address and prefix 64
random_network = IPv6Network((
    ula.network_address + random_address_suffix,
    64))