Net functions behave slightly different for IPv4 from IPv6. Why?

I'm using BigQuery to truncate IPv4 & IP46 IP addresses. By that I mean I want to drop the part that might be used to identify a real person.

Here's some demo code:

select *,
        NET.IP_TO_STRING(NET.IP_TRUNC(NET.IP_FROM_STRING(v.IPv4Address), 24)),
        NET.IP_TO_STRING(NET.IP_TRUNC(NET.IP_FROM_STRING(v.IPv6Address), 64))
from (
  select struct(
     "254.34.78.20" as `IPV4Address`,
     "2a02:c7e:3f0d:e00:48e:abff:d697:9cc2" as `IPv6Address`
  ) as v
)

It returns:

v	f0_	f1_
{ "IPV4Address": "254.34.78.20", "IPv6Address": "2a02:c7e:3f0d:e00:48e:abff:d697:9cc2" }	254.34.78.0	2a02:c7e:3f0d:e00::

I'd simply like to know why these functions return a 0 for the truncated portion of the IPv4 address but nothing for the truncated portion of the IPv6 address.

I know this isn't really a BigQuery question per se as its more about networks and I suspect BigQuery is just doing what any other such library would do... but interested to know why this is nonetheless. Maybe I'll tag it with IPv6 too

Solution

I'd simply like to know why these functions return a 0 for the truncated portion of the IPv4 address but nothing for the truncated portion of the IPv6 address.

It is returning 0 for the truncated part of the IPv6 address.

IPv6 addresses can have several forms, but the canonical form requires you to shorten the longest run of 0 fields with a double colon (::). That is at the end of your address 2a02:c7e:3f0d:e00::, and that means it is 2a02:c7e:3f0d:e00:0:0:0:0 properly shortened.