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Here:
https://docs.python.org/3/library/ipaddress.html#ipaddress.IPv6Network
In the constructor documentation, item 1 says:
"""
A string consisting of an IP address and an optional mask, separated by a slash (/). The IP address is the network address, and the mask can be either a single number, which means it’s a prefix, or a string representation of an IPv6 address. If it’s the latter, the mask is interpreted as a net mask. If no mask is provided, it’s considered to be /128.
For example, the following address specifications are equivalent: 2001:db00::0/24 and 2001:db00::0/ffff:ff00::.
"""
However in issue22800 it has been identified that using the expanded netmask (e.g. ff00::/ff00::) is not supported.