Save user ip in a specific field -- replace user_ip__c with your custom field name - make sure it can store up to 45 characters (for IPv6 addresses)

sf_w2l_user_ip.php

<?php
// Save user ip in a specific field
// -- replace user_ip__c with your custom field name
// -- make that a hidden text field in your lead form (make sure it's enabled)
// -- make sure it can store up to 45 characters (for IPv6 addresses)

add_filter( 'salesforce_w2l_field_value', 'salesforce_w2l_ip_field', 10, 3 );

function salesforce_w2l_ip_field( $val, $field, $form ){
	
	$my_ip_field = 'user_ip__c';
	
	// Target a specific field on all forms
	if( $field == $my_ip_field ){

		if (!empty($_SERVER['HTTP_CLIENT_IP'])) {
		    $ip = $_SERVER['HTTP_CLIENT_IP'];
		} elseif (!empty($_SERVER['HTTP_X_FORWARDED_FOR'])) {
		    $ip = $_SERVER['HTTP_X_FORWARDED_FOR'];
		} else {
		    $ip = $_SERVER['REMOTE_ADDR'];
		}
		
        $val = $ip;
        
	}

    return $val;
    
}