How to play Heroes of Might and Magic III online via Hamachi?

Perhaps the question isn't entirely on-topic, but here's the situation: I've given up trying to fix Hamachi, so I'm playing directly with some people using their external IP addresses. Here's my question: does anyone happen to know why some people can see my IP address (a "white" one, of course), while others can't? (And there's no apparent pattern; the reason isn't simply whether they have a white IP or not).