When working with IP addresses and Prefixes (a.k.a CIDRs) often you have to merge and split address ranges. The current implementation hides for good reasons the internal IP address representation, but then it is unnecessary hard to calculate the last address of a prefix. Within the netip package it is very easy to calc the last address of a prefix.

// last = base | ^netmask

And for symmetry, please also add a netip.Prefix.Base method, even if this is already covered with netip.Prefix.Masked().Addr()