Hi,
If you want a general solution that will work for anybody, you are out
of luck. For example, if the person is on a corporate network or behind
any router (and is thus firewalled), you can't contact them by IP at
all. Further, there is no way to send an email to a person by knowing
their IP. You must know what their email address is, both username and
e-mail host. However, if you can make some assumptions about your users,
you might be able to think of something.
I think in the past, you might have been able to use Windows Messenger
to send messages to arbitrary IP addresses (not sure how it worked, but
it really annoyed me), but I assume that's patched in recent service
packs of Windows, and it's not a reliable way to communicate because
people turn that service off (if they can).
Someone else mentioned the unix program "talk", but I don't think that
is commonly used. I would give up on an automatic solution--if you need
to do this, you probably need to make them log into your site, and get
their email addresses when they first sign up.
Good luck,
Dan
