You're allowed up to 600 queries per hour by default. Once this limit is reached, all of your requests will result in HTTP 444, forbidden, until your quota is cleared.
The HTTP API takes GET requests in the following schema:
https://ip.222.at/{format}/{IP_or_hostname}
Supported formats are: csv, xml, json and jsonp. If no IP or hostname is provided, then your own IP is looked up.
JSON: https://ip.222.at/json/
JSON (opt. w/ Callback): https://ip.222.at/json/8.8.8.8?callback=test