How to find user by an IP address?

How I can find user in my forum if I only have his IP?

Do you mean the location of the user or the user ?

The user.

Well if you want the location where a user is viewing on your forum go to your ACP then below it you will find the "who is online" and all of the online users are display their username , ip address and location on the forum where they view..

I don't mean where is the user, I mean can I find out the nickname of the user with this IP address? Is that even possible?

No possible to input the IP somewhere and it will show who uses it. You will need to go into someones post and click the IP and see if it matches or you can do is check on the homepage in the admin panel and randomly search, but you will need to be online along with the members.