Getting somone's IP address with MSN messenger

hello,

I'm always interested in what happens behind the scenes... This is why I installed a packet sniffer on my computer and started checking out what happens.
When I wanted to retrieve the IP address of the person with whom I was chatting I saw that normal messages go throug a server. While file transfer happes P2P obviously...
Does anyone have any idea how to retrieve the IP address?

thanks,

MuSHi