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Internet Service Providers (ISPs) assign users large IPv6 blocks (~/64) containing millions of IPv6 addresses. If a user was to ping a server using 2 different IPv6 addresses from his given IPv6 block, would the server be able to trace the IPs back to the same Internet user (i.e. same IPv6 block)?

My understanding is that they could easily find out the IPv6 block of the ISP as this information is publicly available, but I'm not so sure about the IPv6 block of the user as this depends on how the ISP decides to split its block internally.

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ISP commonly assign as much as a /56 prefix with 272 addresses to private users (possibly even more).

If you don't incidentally know the specific allocation scheme for an address range, then no, you can't derive a user's address range, you can only guess - it's somewhere between /56 and /64 (4.7 sextillion to 18 quintillion addresses).

Some ISPs reportedly use longer prefixes/smaller ranges in spite of rigid recommendations. Also note that corporate users may be using substantially smaller allocations.

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  • Yes, this is in fact what I was thinking too. I'm going to leave some time for others to answer and if no one comes up with something better I'll upvote this.
    – enriquejr99
    Commented Mar 5, 2021 at 10:42

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