::::Hello again,
::::I think that thanks to [[:en:Classless Inter-Domain Routing|CIDR notation]], it should be easy to block the rest of the range only with a few rules.
::::Consider that CIDR prefixes allow you to block a prefix of bits contained in an address. There are 6 bits of difference between the two prefixes. Hence, you should be able to block only six prefixes to block all addresses except the ones from OVH Telecom.
::::For illustration, here are how both prefixes convert to binary:
::::<syntaxhighlight>Large prefix:
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX (32 digits)
00100000000000010100000111010000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000
= 2001:41d0::

Narrow prefix:
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX (38 digits)
00100000000000010100000111010000111111000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000
= 2001:41d0:fc00::
</syntaxhighlight>
::::You should just have to truncate the narrow prefix at different points, then  flip the last bit that comes just before the end of the prefix to block a range that pertains to the large but NOT the narrow concerned range.
::::I have written a small Python script that does this automatically for the example.
::::<syntaxhighlight lang="Python">#!/usr/bin/python3
#-*- encoding: Utf-8 -*-
from ipaddress import IPv6Network, IPv6Address
from argparse import ArgumentParser

args = ArgumentParser(description = 'Find how to block a large IPv6 range excluding a narrow IPv6 range only while blacklisting a few prefixes.')

args.add_argument('narrow_prefix')
args.add_argument('narrow_prefix_size')
args.add_argument('large_prefix_size')

args = args.parse_args()

narrow = IPv6Address(args.narrow_prefix)

IPV6_BIT_SIZE = 128

for intermediary_size in range(int(args.narrow_prefix_size) + 1, int(args.large_prefix_size) + 1):

    mask = ((1 << intermediary_size) - 1) << (IPV6_BIT_SIZE - intermediary_size)

    bit_to_invert = (1 << (IPV6_BIT_SIZE - intermediary_size))

    print(str(IPv6Address((int(narrow) & mask) ^ bit_to_invert)) + '/' + str(intermediary_size))

</syntaxhighlight>
::::You can execute it with :
::::<syntaxhighlight>./generate_addresses.py 2001:41d0:fc00:: 32 38</syntaxhighlight>
::::Here is what the concerned script outputs:
::::<syntaxhighlight>2001:41d0::/33
2001:41d0:8000::/34
2001:41d0:c000::/35
2001:41d0:e000::/36
2001:41d0:f000::/37
2001:41d0:f800::/38
</syntaxhighlight>
::::Do you think that it is okay to block these only six ranges?
::::Regards, --~~~~