How can we know the IP of a person we are chatting with using
Yahoo! Messenger ??

How come you need their I.P?

Usually the port number is "4443" sometimes it doesn't work so i am guessing it is because that the person has a firewall. So here is the script.

open notepad and enter the following:

@echo.
@color 09
@netstat -n
@echo.
@pause

then save as "GetIp.cmd" and when opened you will view all established connection

so your looking for xx.xx.xx.xx:4443

if your behind a firewall the port is whichever was free