Logging IP with Guest Entries Plugin
You could add a field "IP Address" to your Entry Type and make the form submit the guest's IP (→ getIpAddress() method) via a hidden field.

&lt;input type="hidden" name="fields[ipAddress]" value="{{ craft.request.getIpAddress() }}"&gt;


For further improvements you could make your own field type for "IP Address" with a plugin, that adds custom validation (→ block certain IP ranges). You could also hide the field from the CP or make it only readable.