Getting Ip from a forum

Hi,

I know very little about computers in general, thats why i searched this site to ask a question.

basicly, i want to know if there is a way to identify a user on a discussion forum, just to know who it is.

so, say you are in a forum and you are coming across somebody you think might just be an old user with a new name...and you'd like to know.

can their ip be matched up from their original users posts to the new ones? if so, how would you do this?

thanks