MySql uses SQL, which is a querying language, thus can't store variables. You are going to need something to 'get' the IP of the user, and use whatever you used (I'll say you'll be using PHP) to query your database with the information gathered.
for example in PHP:
$ip = $_SERVER['REMOTE_ADDR'];
then your SQL query:
SELECT * FROM [Table] WHERE Ipaddress = '$ip';
If I understand you right, you are trying to do something that is beyond plain Structured Query Language.