Getting the IP address of each time the user has logged in and save it in profile ....

Hello there ..
 
I have profile variable called IP Address ,
i need this Profile to add Unique IP address that has been used by the UserName ,,,
meaning :
If i logged in for the first time , it will save my IP address , next time if my IP has changed and i logged in , i need to keep the old IP address and add to it with comma seperated the new IP ..
 
Can you help me in that ... 
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