On the original 2channel, I've heard that the IP address is only logged in plaintext for the first post in a thread.

The ID is obviously some kind of hash of a thread ID and an IP address. And that means the admin ostensibly can't just extract the IP address from the ID. But, given a specific thread and a selection of possible IP addresses, could he tell whether that post came from that IP address?

Also, why does sage hide the ID?