To use the end user's IP address as a default value for a Salesforce Submit field, enter the following variable into the Default Value field in the Salesforce Element popup:

{$IPAddress}

A common use for including the IP address in the form submission would be for petition forms where you wished to collect the IP address as documentation that the signatures on the petition were from actual web users.

If you include this, it is good practice to make this Salesforce Element a hidden field so as not to confuse the end user.