ResponseFull is hosted on AWS. AWS assigns dynamic IP addresses, so our external-facing web application IP address can and will change. That being said, AWS provides their IP address ranges if you need to white-list our application. At the time of writing, this list is provided in JSON and is kept up-to-date by Amazon.
If you have SalesForce integrations enabled, the ResponseFull communication to SFDC originates from 18.104.22.168.