When a contact form is submitted, the IP FORM plugin records the client's IP with the POST data.
The following line showing the IP used to submit the contact form is inserted at the end of the message:
Sent from: xxx.xxx.xxx.xxx
This is useful to track IPs used to submit request through contact forms and eventually block spammers from sending more messages.
The plugin doesn't take any parameters. Just install it and enable it from the plugin manager in the backend.