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Exactly enough to fill out the address, which is always the same length. BTW, IPv4 does basically the same thing. The address 127.1 is equivalent to 127.0.0.1.

Not really the same, the mechanics are different and this particular behaviour is pretty much an accident, not abbreviation.

In IPv4 you also have 127.257 equal to 127.0.1.1, 123456789 equal to 7.91.205.21, and 010.010.010.010 is a well-know DNS server. This notation is also rejected by most implementations.

It is? Those alternate IPv4 notations are all accepted by Linux, FreeBSD, and MacOS. I remember playing around with "alternate notations" 30+ years ago on old SunOS boxes.

But IPv6 is "too hard"

There are a total of 8 groups of 4 hex digits, so 8 minus however many groups you already have.

google.com: 2607:f8b0:4009:819::200e (5 groups) -> 2607:f8b0:4009:0819:0000:0000:0000:200e (3 groups of added zeros)

a ULA address: fd2a:1::2 (3 groups) -> fd2a:0001:0000:0000:0000:0000:0000:0002 (5 added)

localhost: ::1 -> 0000:0000:0000:0000:0000:0000:0000:0001

However many are left. In what circumstances do you care?

However many it takes to make the whole A::B number exactly 128 bits long.

“Enough”