[bibaheu]

Probably IPv4 is on CGNAT and Sprint doesn't keep the logs of the translation. On IPv6 there's no NAT, and there might be a deterministic relationship between subscription and IP

[glogla]

That, or they don't give devices IPv4 addresses at all and run 464XLAT - according to Wikipedia, quite a few telcos do it that way.

[p1mrx]

464XLAT is a form of CGNAT.

The main difference is whether the subscriber side uses an IPv6 or private IPv4 address, but on the internet side they are equivalent.

[keneda7]

I believe you are correct.

https://news.ycombinator.com/item?id=16440850