how to track IP Adress of User

hell,
Actually i need to track IP Adress for secuity and Requirment of my project. So anybody can tell how i can get IP Adress of user?
I am developing a module on that user can just register only one time for one Specific IP Address. User cant register 2nd time from Same IP Adress.
So for that i need to Trace IP Adress of Register user.


Thanks in advance


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