Private IP address with yahoo messenger

[Alcides <alcides.hercules () gmail com>]

Hi list,
I was wondering to have any clue on this.
While having a chat session with somebody in yahoo messenger, how is
it possible to obtain his/her Private IP address if that person is
behind a Router and/or a Firewall (NATed)?
Moreover,
Is there any way that he/she can hide successfully (behind the NAT or
something else)so that NO internal IP address is revealed?
Thanks.
Warm regards.