Error in field form_hidden

[eluser]Unknown[/eluser]
Form_hidden box is where I want to put an IP

Code:

<?php echo form_hidden('ip_address', $REMOTE_ADDR);?>

and is an error:

A PHP Error was encountered
Severity: Notice
Message: Undefined variable: REMOTE_ADDR
Filename: views/form.php
Line Number: 19


What am I doing wrong?